Hello,
@jarod5001 is right, I faced the same problem two weeks ago, and solved it last week thanks to Yunohost Forum.
You just have to edit the file /usr/lib/moulinette/yunohost/data_migrations/0015_migrate_to_buster.py
# if upgradable_system_packages:
# raise YunohostError("migration_0015_system_not_fully_up_to_date")
then retry an update
No need to backup you system, Yunohost will do it for you.