It it impossible to add relationships to a non-label from a label edit page. Try, e.g., https://beta.musicbrainz.org/label/f669a7a4-b94b-42db-a0b7-ba040e5c30d8/edit - click on the "Add relationship" button at the top, choose "Artist → founders", select any artist to relate to, and click done: Nothing happens, the relationship is not added. Try relating to a label instead, and it works (whether the specific relationship type is already used or not).
(This ticket is split from
MBS-7563, because they are technically unrelated.)