I think according the core book it only set the minimum ac to 16 and it dont forbid to give on the top other bonuses.
I suspect you're not grasping the nuance between "cannot be lower than 16" and "sets score to 16"; in english, these terms
do not mean the same thing. Barkskin does
not "set your score to 16" - it only makes it so that it "cannot be lower than 16". These phrases have very
different meanings, but you seem to be thinking in a way that treats them as meaning the same thing. If there is a language barrier at work here, and you're not understanding the nuance of what is being said, let me know and I'll try again.
As per the written rules, in core 5e, quoted directly from the 5e Players Handbook (above), the spell works as the examples I've described. If you don't understand
why that is the case, especially if english language is a hurdle, I can try to break it down into simpler language.
