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:

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if it is ticked untick it then scroll to the bottom of the page and save.

Now try deleting it again.