Can't fix Paypal "invalid_client" authentication error - paypal

Question

I'm trying to test my Paypal integration from localhost using Paypal Sandbox, but whenever I try to get an access token from Paypal, I get back the following error:
{
"error": "invalid_client",
"error_description": "Client Authentication failed"
}
I make the request with curl:
curl -v https://api.sandbox.paypal.com/v1/oauth2/token \
-H "Accept: application/json" \
-H "Accept-Language: en_US" \
-u "clientId:secret" \
-d "grant_type=client_credentials"
Obviously I am using the actual client id and secret in the request, and I am SURE I am using the correct client id and secret for the sandbox environment (I double checked).
Curl's verbose logs look like this:
curl -v https://api.sandbox.paypal.com/v1/oauth2/token \
> -H "Accept: application/json" \
> -H "Accept-Language: en_US" \
-u "clientId:secret" \
> -d "grant_type=client_credentials"
* Trying 173.0.82.78...
* TCP_NODELAY set
* Connected to api.sandbox.paypal.com (173.0.82.78) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:#STRENGTH
* successfully set certificate verify locations:
* CAfile: /etc/ssl/cert.pem
CApath: none
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Request CERT (13):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Certificate (11):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server did not agree to a protocol
* Server certificate:
* subject: C=US; ST=California; L=San Jose; O=PayPal, Inc.; OU=PayPal Production; CN=api.sandbox.paypal.com
* start date: Jul 27 00:00:00 2020 GMT
* expire date: Aug 1 12:00:00 2022 GMT
* subjectAltName: host "api.sandbox.paypal.com" matched cert's "api.sandbox.paypal.com"
* issuer: C=US; O=DigiCert Inc; OU=www.digicert.com; CN=DigiCert SHA2 High Assurance Server CA
* SSL certificate verify ok.
* Server auth using Basic with user 'clientId' // The original log has the actual clientId here
> POST /v1/oauth2/token HTTP/1.1
> Host: api.sandbox.paypal.com
> Authorization: Basic QVRQZGl5TTFqeXpQZkdxMW9FQ2xnZG1uUE13VDRQM01yZEJ1MGRGWVZGLWRNVjk1X09xVjZVam1nNU5xRC0wR2ZPT1I3U1ZianYybWQzQXc6RURQSVdoREpPM0dkdndYNVlRaG45QzRCYzBlUTctaEpWZ041ZlVjUlpkbG1Ta0pMelhzZXE0TmpXeEZMV3kwcGdBNE5zTE5MZ1Qyc0pBVW0=
> User-Agent: curl/7.54.0
> Accept: application/json
> Accept-Language: en_US
> Content-Length: 29
> Content-Type: application/x-www-form-urlencoded
>
* upload completely sent off: 29 out of 29 bytes
< HTTP/1.1 401 Unauthorized
< Cache-Control: max-age=0, no-cache, no-store, must-revalidate
< Content-Length: 77
< Content-Type: application/json
< Date: Wed, 07 Apr 2021 08:57:50 GMT
< Paypal-Debug-Id: 891f84363e502
< X-Paypal-Token-Service: IAAS
<
* Connection #0 to host api.sandbox.paypal.com left intact
{"error":"invalid_client","error_description":"Client Authentication failed"}
The problem seems to be here:
ALPN, server did not agree to a protocol
I have NO idea what it means. Could this have something to do with the fact that I'm testing from localhost? How can I fix this?

Answer 1

Decoding the base64 in the verbose log, the client ID passed is ATPdiyM1jyzPfGq1oEClgdmnPMwT4P3MrdBu0dFYVF-dMV95_OqV6Ujmg5NqD-0GfOOR7SVbjv2md3Aw
This does not work when testing at https://developer.paypal.com/demo/checkout/#/pattern/client , so it is not a valid ID.
( Link showing the error, "client-id not recognized for either production or sandbox" )
Create a new REST app, https://www.paypal.com/signin?intent=developer&returnUri=https%3A%2F%2Fdeveloper.paypal.com%2Fdeveloper%2Fapplications