Google cloud Kubernetes deployment error: Field is immutable - kubernetes

Question

After fixing the problem from this topic Can't use Google Cloud Kubernetes substitutions (yaml files are all there, to not copy-paste them once again) I got a new problem. Making a new topic because there is the correct answer for the previous one.
Step #2: Running: kubectl apply -f deployment.yaml
Step #2: Warning:
kubectl apply should be used on resource created by either kubectl
create --save-config or kubectl apply
Step #2: The Deployment
"myproject" is invalid: spec.selector: Invalid value:
v1.LabelSelector{MatchLabels:map[string]string{"app":"myproject",
"run":"myproject"},
MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is
immutable
I've checked similar issues but hasn't been able to find anything related.
Also, is that possible that this error related to upgrading App Engine -> Docker -> Kubernetes? I created valid configuration on each step. Maybe there are some things that were created and immutable now? What should I do in this case?
One more note, maybe that matters, it says "kubectl apply should be used on resource created by either kubectl create --save-config or kubectl apply" (you can see above), but executing
kubectl create deployment myproject --image=gcr.io/myproject/myproject
gives me this
Error from server (AlreadyExists): deployments.apps "myproject" already exists
which is actually expected, but, at the same time, controversial with warning above (at least from my prospective)
Any idea?
Output of kubectl version
Client Version: version.Info{Major:"1", Minor:"14", GitVersion:"v1.14.7", GitCommit:"8fca2ec50a6133511b771a11559e24191b1aa2b4", GitTreeState:"clean", BuildDate:"2019-09-18T14:47:22Z", GoVersion:"go1.12.9", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"13+", GitVersion:"v1.13.11-gke.14", GitCommit:"56d89863d1033f9668ddd6e1c1aea81cd846ef88", GitTreeState:"clean", BuildDate:"2019-11-07T19:12:22Z", GoVersion:"go1.12.11b4", Compiler:"gc", Platform:"linux/amd64"}
Current YAML file:
steps:
- name: 'gcr.io/cloud-builders/docker'
entrypoint: 'bash'
args: [
'-c',
'docker pull gcr.io/$PROJECT_ID/myproject:latest || exit 0'
]
- name: 'gcr.io/cloud-builders/docker'
args: [
'build',
'-t',
'gcr.io/$PROJECT_ID/myproject:$BRANCH_NAME-$COMMIT_SHA',
'-t',
'gcr.io/$PROJECT_ID/myproject:latest',
'.'
]
- name: 'gcr.io/cloud-builders/kubectl'
args: [ 'apply', '-f', 'deployment.yaml' ]
env:
- 'CLOUDSDK_COMPUTE_ZONE=<region>'
- 'CLOUDSDK_CONTAINER_CLUSTER=myproject'
- name: 'gcr.io/cloud-builders/kubectl'
args: [
'set',
'image',
'deployment',
'myproject',
'myproject=gcr.io/$PROJECT_ID/myproject:$BRANCH_NAME-$COMMIT_SHA'
]
env:
- 'CLOUDSDK_COMPUTE_ZONE=<region>'
- 'CLOUDSDK_CONTAINER_CLUSTER=myproject'
- 'DB_PORT=5432'
- 'DB_SCHEMA=public'
- 'TYPEORM_CONNECTION=postgres'
- 'FE=myproject'
- 'V=1'
- 'CLEAR_DB=true'
- 'BUCKET_NAME=myproject'
- 'BUCKET_TYPE=google'
- 'KMS_KEY_NAME=storagekey'
timeout: 1600s
images:
- 'gcr.io/$PROJECT_ID/myproject:$BRANCH_NAME-$COMMIT_SHA'
- 'gcr.io/$PROJECT_ID/myproject:latest
deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: myproject
spec:
replicas: 1
selector:
matchLabels:
app: myproject
template:
metadata:
labels:
app: myproject
spec:
containers:
- name: myproject
image: gcr.io/myproject/github.com/weekendman/{{repo name here}}:latest
ports:
- containerPort: 80

Answer 1

From apps/v1 on, a Deployment’s label selector is immutable after it gets created.
excerpt from Kubernetes's document:
Note: In API version apps/v1, a Deployment’s label selector is
immutable after it gets created.
So, you can delete this deployment first, then apply it.

Answer 2

The MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is immutable because it is different from your previous deployment.
Try looking at the existing deployment with kubectl get deployment -o yaml. I suspect the existing yaml has a different matchLables stanza.
Specifically your file has:
matchLabels:
app: myproject
my guess is the output of kubectl get deployment -o yaml while have something different, like:
matchLabels:
app: old-project-name
or
matchLabels:
app: myproject
version: alpha
The new deployment cannot change the matchLabels stanza because, well, because it is immutable. That stanza in the new deployment must match the old. If you want to change it, you need to delete the old deployment with kubectl delete deployment myproject.
Note: if you do that in production your app will be unavailable for a while. (A much longer discussion about how to do this in production is not useful here.)