Why? I try to do it but I can't

You need to hold ALT while using pick block

I know that the button needs to be hold, I do it, the fact is that it copies all the blocks, but there is no side of stairs

yes

I'm not entirely sure what you mean. Are you trying to copy the orientation of the side of the stairs? Are you also in creative mode?

Are you sure the mod is loaded? Do you have Mod Menu installed, does it show Pick Block Pro inside the mod list?

What happenens when you place the block down?

nothing

I wanted to add this, yes, the mod is installed, it works with everything else except this one, mod on the list

Does it say "+BlockState NBT" when you hover over the item inside the inventory?

no
I am using the latest version fabric 1.20.1
I hold alt when I do this.

What else did you try that did actually work?

Stairs are one of the first things I test on every new mc version and they work perfectly fine for me:

Ah lol, that's alright, that can happen. I'm happy to hear it's working correctly though :)
I'll close this now

oh my god.. i am now feeling cringe from myself and shame.. i pressed ctrl instead of alt literally all the time, confusing them and not noticing the difference.. I'm sorry that I wasted your time in my stupidity