Below you can see that testing for external testers are turned off:
same for internal testing:
but when I transfer app, one of the criterias are not me
I struggled with this for an entire day, I tried everything and finally found another workaround:
1) Expire all the builds
2) Delete all testers
3) Clear all the informations in the Testflight tab
4) Delete all langages except the main one
And finally the step which fixed the Testflight criteria for us
5) Create a new app version (no need to submit or even edit it)
now you check transfer.
Good Luck.