Paradox released a small hotfix on October 6th: fixed an interaction bug after using artifacts that made it impossible to assign training to children.
The full update 1.20.1 is still in development at the time of publication. Therefore, the current hotfix should not be passed off as a ready-made large patch. If the problem persists, the developers suggest testing the game without modifications and submitting a report through the bug forum. When checking, indicate the version and specific interaction that was performed before the available recipient disappeared.
Sources and checks
Update 1.20.0.4 · Steam