The general advice is that it’s better to export the strings to a .po file, edit them using something like PoEdit, then import them again.
However if you want to edit them directly in UE and you find you have this error, I’ve found you can fix it just by copying the Game.archive file from your base language directory (e.g. en) into your newly created language folder. It will be misleading because it’ll think all the translations are Complete, but it does at least solve the problem and you can edit the translations in UE after that.