The app couldn't be installed because developer certificate is no longer valid
I encountered this error because the build was signed using a provisioning profile whose certificate had been revoked.
When i checked in the developer center, the provisioning profile was marked as "Invalid".
I just clicked "Edit", selected the new developer certificate and regenerated the provisioning profile. When the new build was uploaded on testflight signed with the new provisioning profile, the error went away.
However, i do think Apple should be checking if the provisioning profile is still valid rather than developers finding it out after build is already approved by TestFlight and testers complain about this error.
It simply means that the developer account from which this app is uploaded is expired OR the development/distribution certificates of the app are expired or revoked. It can be resolved by activating the certificates of the app on apple's developer portal.
We have uploaded a version to apple-test flight and apple approved app for external testers. Now we're trying to download and is not working, got same message.
But if we run app from dev phone push notification works so P12 still valid. And distributed with old test flight process works.
We're trying to upload another version and see if works.