We've developed an add-on, but whenever we try to install it we get an 'Invalid add-on' message. This is when adding the ID directly into the Gmail 'Add-ons' tab.
I've copy-pasted it into another G Suite account, and it works fine. So I'm assuming the problem can't be with the code - it must be our account somehow.
Has anyone come across this? I've checked every API access setting I can think of.
Solved this: we had marketplace apps set to whitelist-only, which apparently affects add-ons. Instructions for disabling are here: https://support.google.com/a/answer/6089179?hl=en.