I can't add my friends in genshin even if I and my friends reached level 16 and we are the same country..

We already have an answer for a similar question which you can see here:
I can’t add or find my friends even if I enter their UID correctly.?

If you have more answers to add you can do that below.

You don’t actually need to reach AR16 to add friends. What you do need is :

1) You’re friend’s UID

2) You’re friend’s approval of the friendship request