how can i buy a free game, the buttton to buy the game isn't highlighted, plz help

What's the free game in question?
Is it by any chance a DLC to a game you don't own here on GOG?

Because in that case there should be a message on the screen, saying something like:

Please note that you are required to own [game X] on GOG.com to be able to purchase and play [DLC X].

Edit: attached

It's "Neverwrites: enhansed edition

I assume, that's supposed to read "Neverwinter Nights: Enhanced Edition"?

The point stays the same: you have to own the base game to get the free Extras.

Edit: I guess the attached picture settles that issue once and for all.

Well, if you're looking for the actual game, it isn't free. I assume you're looking at one of the free language extras, which contain the in-game movies with subtitles. And, as pointed out, you can't get those unless you own the base game from here.