If the column doesn't exist anymore, you could delete the migration entry from the migrations table and Flarum will stop trying to roll it back and will run it again on the next enable (based on the error message, it's probably the 2022_02_09_000000_add_posted_on_to_posts migration).
Ideally, the extension developer should write the "down" migrations so that it gracefully skips if the column/table already doesn't exist ($schema->hasColumn($table, $column) can be used).
