It all starts from this error…
Yes, you can click the button and (possibly) fix it, but what was the cause of that?
The answer is simple: every Service Connection you are going to consume within a Pipeline needs to be authorised. What usually happens is that a Service Connection is automatically authorised for the project (or even across projects), and then someone decides to remove the pipeline authorisation leaving you with a red build.
Fixing this is really easy - your Service Connection has got a Security menu. Look into it…
Click on the + button to add at least a pipeline, or to grant full-fledged access across different projects:
Needless to say, you can restrict access as much as you like - play with the ACLs as much as you like, but regardless of that Pipelines and Users will have different authorisation models. Especially because Pipelines themselves are considered resources in the new YAML world…