I am trying to publish Gmail Addon which I create before. I follow that documentation https://developers.google.com/gmail/add-ons/how-tos/publish#publish_an_add-on. According the documentation I am reaching the API Console from Resources > Cloud Platform project. And install the Google Apps Marketplace SDK. I cannot save Configuration because of extension selection. I cannot select Gmail Addon extension checkbox. You can see the image 
Do you have any idea how I can solve this?
Cheers.
This is a bug on that page. (can't save "Apps Marketplace SDK" Configuration for Gmail Addon). I have reported it here
Please, add yourself to "I am also facing this issue" list so that the issue gets noticed by the Google developers.
Thanks for the report. This is due to an issue in the UI, we already have a fix for and should be rolled out soon. Please note that currently to publish a Gmail add-on the listing has to be domain private or whitelisted, see https://developers.google.com/gmail/add-ons/how-tos/publish.
I think you can click Gmail add on extension now. But after click and fill up Deployment ID,I can't understand how to ''Fill in the Visibility subsection by selecting Public or Private visibility for your add-on.''
来源：https://stackoverflow.com/questions/49752787/publish-gmail-addon-select-extension