It's hard to say what the best way forward is.
If the database was fine before migrating to the new server, the best course of action might be to start the migration again if possible.
Otherwise you'll have to take a look at what exactly is broken in the database. Which tables are missing, or are any columns missing in existing tables. You could compare this by making a new Flarum installation, or by manually reading each migration and checking whether the migration is applied to your database.
Then you'll need to insert or remove the name of the migrations together with the extension ID in the migrations table.
If the data in those tables aren't important, you could also delete the existing tables that Flarum complains already exist, and let Flarum create them again. You'll lose all existing tags and the discussions they were connected to.
It's odd to be missing migrations for the Tags extension, because it's one of the extensions that are installed automatically when you install Flarum. For the database to be broken like this, something must have gone wrong at some point, either during the export or import of the database, or maybe you tried uninstalling the Tags extension and the server crashed in the middle of the process.
If you're able to go back to an earlier backup, it'll probably be the most reliable solution as you will be sure anything wrong has been rolled back.
