Error message when merging contacts
This happened to us, and it turned out that we didn't have a "supervised" dedupe rule defined for individuals. You can have at most one "supervised" and one "unsupervised" dedupe rule. Our supervised rule had disappeared (though I haven't worked out how). I recreated it and the error disappeared.
There's some relevant documentation here: https://docs.civicrm.org/user/en/latest/common-workflows/deduping-and-merging/#dedupe-rules