Git commit 370a22daaded23f4a754fb5168cb582ed34a9d14 ("flatpak: handle end_of_lifed_with_rebase") introduced use of flatpak_transaction_add_rebase_and_uninstall but that's only available from 1.15.4 on. Either discover needs to raise the minimum required version or use separate rebase + uninstall calls.
*** This bug has been marked as a duplicate of bug 487404 ***