Undefined array key “compare” in lib/search.php

Yeah, we’ll fix that in the next version. It’s annoying to get the warning, but it’s nothing dangerous. Defining the compare in the meta query you’re using will make the warning go away.

No, the warning clearly says it’s because the compare parameter is not set (it’s an optional parameter, and Relevanssi should treat it like that but doesn’t). That’s why setting it will make the warning go away. The relation parameter is not related.