Client Characteristic Configuration descriptor 0x81 improperly configured - swift

Question

I can't write to the RACP record to start the appropriate procedure on the glucose device and I don't know what's wrong:
Bluetooth powered on.
Device Contour7802H6798544 found, try to connect...
Connected to Contour7802H6798544
*******************************************************
Service: <CBService: 0x14566740, isPrimary = YES, UUID = Glucose>
<CBCharacteristic: 0x1466fdc0, UUID = 2A18, properties = 0x10, value = (null), notifying = NO>
... has notify
<CBCharacteristic: 0x1465bb00, UUID = 2A52, properties = 0x28, value = (null), notifying = NO>
... has write
*******************************************************
Characteristic's value subscribed
Subscribed. Notification has begun for: 2A18
Optional(Error Domain=CBATTErrorDomain Code=129 "Unknown ATT error." UserInfo={NSLocalizedDescription=Unknown ATT error.})
129 is 0x81 in HEX, the specification of the error message in the title can be found in the official Bluetooth docs of the Glucose Service: https://github.com/www378660084/BluetoothProfile/blob/master/GLS_SPEC_V10.pdf
I write op-code and operand to 0x2a52 as follows:
let data = Data([0x04,0x01]);
peripheral.writeValue(data , for: racp?, type: .withResponse);
And this is the specification of the characteristic:
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio Designer Edition 9.1.10.3537 (http://www.liquid-technologies.com) -->
<!--Copyright 2011 Bluetooth SIG, Inc. All rights reserved.-->
<Characteristic xsi:noNamespaceSchemaLocation="http://schemas.bluetooth.org/Documents/characteristic.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
name="Record Access Control Point"
type="org.bluetooth.characteristic.record_access_control_point"
uuid="2A52" last-modified="2011-12-6" approved="Yes">
<InformativeText>
<Summary>This control point is used with a service to provide
basic management functionality for the Glucose Sensor patient
record database. This enables functions including counting
records, transmitting records and clearing records based on
filter criterion. The filter criterion in the Operand field is
defined by the service that references this characteristic as
is the format of a record (which may be comprised of one or
more characteristics) and the sequence of transferred
records.</Summary>
</InformativeText>
<Value>
<Field name="Op Code">
<Requirement>Mandatory</Requirement>
<Format>uint8</Format>
<Enumerations>
<Enumeration key="0"
value="Reserved for future use (Operator:N/A)" />
<Enumeration key="1"
value="Report stored records (Operator: Value from Operator Table)" />
<Enumeration key="2"
value="Delete stored records (Operator: Value from Operator Table)" />
<Enumeration key="3"
value="Abort operation (Operator: Null 'value of 0x00 from Operator Table')" />
<Enumeration key="4"
value="Report number of stored records (Operator: Value from Operator Table)" />
<Enumeration key="5"
value="Number of stored records response (Operator: Null 'value of 0x00 from Operator Table')" />
<Enumeration key="6"
value="Response Code (Operator: Null 'value of 0x00 from Operator Table')" />
<ReservedForFutureUse start="7" end="255" />
</Enumerations>
</Field>
<Field name="Operator">
<Requirement>Mandatory</Requirement>
<Format>uint8</Format>
<Enumerations>
<Enumeration key="0" value="Null" />
<Enumeration key="1" value="All records" />
<Enumeration key="2" value="Less than or equal to" />
<Enumeration key="3" value="Greater than or equal to" />
<Enumeration key="4" value="Within range of (inclusive)" />
<Enumeration key="5"
value="First record(i.e. oldest record)" />
<Enumeration key="6"
value="Last record (i.e. most recent record)" />
<ReservedForFutureUse start="7" end="255" />
</Enumerations>
</Field>
<Field name="Operand">
<InformativeText>The operands correspond to the Op Code
values (Keys 0 to 255) defined in the Op Code Field
above</InformativeText>
<Requirement>Mandatory</Requirement>
<Format>variable</Format>
<Enumerations>
<InformativeText>Op Code / Operand Value
Correspondence</InformativeText>
<Enumeration key="0" value="N/A" />
<Enumeration key="1"
value="Filter parameters (as appropriate to Operator and Service)" />
<Enumeration key="2"
value="Filter parameters (as appropriate to Operator and Service)" />
<Enumeration key="3" value="Not included" />
<Enumeration key="4"
value="Filter parameters (as appropriate to Operator and Service)" />
<Enumeration key="5"
value="Number of Records (Field size defined per service)" />
<Enumeration key="6"
value="Request Op Code, Response Code Value" />
<ReservedForFutureUse start="7" end="255" />
</Enumerations>
<AdditionalValues>
<InformativeText>Response Code Values</InformativeText>
<Enumeration key="0" value="Reserved For Future Use"
description="N/A" />
<Enumeration key="1" value="Success"
description="Normal response for successful operation" />
<Enumeration key="2" value="Op Code not supported"
description="Normal response if unsupported Op Code is received" />
<Enumeration key="3" value="Invalid Operator"
description="Normal response if Operator received does not meet the requirements of the service (e.g. Null was expected)" />
<Enumeration key="4" value="Operator not supported"
description="Normal response if unsupported Operator is received" />
<Enumeration key="5" value="Invalid Operand"
description="Normal response if Operand received does not meet the requirements of the service" />
<Enumeration key="6" value="No records found"
description="Normal response if request to report stored records or request to delete stored records resulted in no records meeting criteria." />
<Enumeration key="7" value="Abort unsuccessful"
description="Normal response if request for Abort cannot be completed" />
<Enumeration key="8" value="Procedure not completed"
description="Normal response if unable to complete a procedure for any reason" />
<Enumeration key="9" value="Operand not supported"
description="Normal response if unsupported Operand is received" />
<ReservedForFutureUse start="10" end="255" />
</AdditionalValues>
</Field>
</Value>
<Note>The fields in the above table are in the order of LSO to
MSO. Where LSO = Least Significant Octet and MSO = Most
Significant Octet.</Note>
</Characteristic>
0x04 0x01 means I want to get the total number of stores records.
But I get always the error message mentioned above.

Answer 1

I used to have the same issue, but after checking the spec I found this:
Ensure that the client configures RACAP Charac for indication
by writing "0x0200" on the Client configuration descriptor related to RACAP
From section 3.4 of the specification.