Found this old thread and wanted to provide an answer, because it stumped us for a bit.
If you want to transfer an app to another account, you may be getting stopped by "You must first remove all builds and testers from the app you want transferred and clear each information field below Test Information." You can't remove the builds. But it's the second part that matters. You need to delete the info in a few fields that are not obvious.
In the App Store Connect entry for your app, go to the Test Flight tab, then under the General Information entry in the sidebar, click on Test Information.
On the following screen, delete anything that's in any of those fields, then click the Save button.
You should now be able transfer the app without that roadblock.
We got hung up on the idea that we needed to delete the expired TestFlight builds. In truth, it was just deleting text out of some random fields.