I was on https://bookbrainz.org/work/f4fc0f56-54b3-45ed-b4a6-a204760ff524/relationships and added the relationship of the work to its author without clicking the "Add relationship" button before hitting "submit". This resulted in an empty error bar (will create another ticket for it). I think that for simply adding a single relationship, clicking the "Submit" button with just the "Add relationship" section filled out should work. It's also a bit weird that after clicking "Add relationship" the relationship moves to the "current relationships" section, but in fact has not yet been submitted.

The current behaviour would be more easy to understand with different section titles, like "New relationship" and "Relationships to submit" or something like that.