There is no reason for this error, as Adam mentioned.
You will need to check the logs on the instances themselves or the SSH in them, see what happens, or check the Builder logs that will be provided when you run the deployment command, and which you can also find in Logs Viewer by selecting “Container Builder” as the resource for view logs (the default is App Engine by default).
A common reason is not the implementation of a 200 response handler for /_ah/health , but there are other reasons. Perhaps simply because it will take a long time for the instance to appear for various reasons, and it is completely healthy. The deployment team simply refused to poll their status.