I believe (but could be incorrect) that even if the target has a higher AC it will lower it to 16 if you cast barkskin.
Fortunately it doesn’t, and as far as I can tell is working as per 5e, where its main use case past the first few levels seems to be buffing AC of wildshapes, animal companions and summons who can’t wear armour or items, rather than increasing AC of characters who already are hard to hit.
See
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=840670&page=1 for more discussion on this topic, including some screenshots I took showing that it doesn’t reduce AC that is over 16, only raises it to 16 if it isn’t already.
