I understand your issue is:
I can not access my instance via terminal.
Server refused our key
The key is already in the instance, the key was generated via ssh from the instance ssh window.
This IAM is complicated a lot of information in the documentation, it seems that leads nowhere.
If you have a different concern, please restate your message in English and
provide additional details for us to address your request correctly.
To connect to an App engine flex by terminal you can do it with the gcloud with the following command:
gcloud beta app instances --project [PROJECT-ID] ssh [INSTANCE-NAME] --service [SERVICE] --version [VERSION]
I hope this can work for you, and I leave the documentation[1] where it is better explained. This way you don't need the key to communicate to your instance as the gcloud command will manage the authentication.