Unable to delete VM instance. Delete button is disabled
Sounds like you have enabled deletion protection in Google Cloud Platform. To check this go to; Computer Engine > VM instances, and then click on the instance you are having the issue with.
Up the top click the edit button > scroll down the page a little bit to Deletion protection, make sure it is unticked:
if it is ticked untick it then scroll to the bottom of the page and save.
Now try deleting it again.