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I have had an app with a custom domain hosted on Google App Engine for several years. I'm finally getting around to enabling managed security, and it won't work for me. I select the custom domain and the Enable managed security button remains disabled. Mousing over the button shows the message "You must be a domain owner to enable security..."
screenshot of custom domains tab
What should I try next?
Make sure that your account is still domain owner or is the same that you used to map the custom domain years ago.
Also, you could try to verify again your domain ownership at Webmaster, I recommend that you use the txt record verification method.
google-cloud-platform google-app-engine or ask your own question. 
