personal income tax codes

If you did not do your accounting in Dexxter in the previous year, then Dexxter will indeed not be able to help you add the codes. You must add codes to your tax return if you were self-employed in the previous year. That is part 2 of your tax return and depends a little on what you did. So you will have to add codes, but Dexxter will not be able to tell you what they are because you did not do your accounting in Dexxter.

I can't see what it states there, because I haven't added the 2021 accounts from Dexxter yet. (Among other things, so that I have everything in Dexxter).

If I can help, I'm happy to do so. Let me know.