can't click to " pay for your order now "

Which browser?
Which game?
Which payment option?
Gift option selected?

Chrome
Darkest Dungeon: The Color Of Madness
Paypal

It means you are not supposed to buy it.

That's a DLC and you can't buy DLC's unless you own the original game.

thanks you , i'm stupid :v can i ask you one more thing , how to get redeem code ? ( i come to GOG first time )

Click "Redeem code" link at the bottom of the page or use this link.

This is DLC which requires a base game to work.

To buy DLC here, you need to own the game here. Likewise you can buy a giftcode, but only use it, if you use the base game here.

So if you own the base game on Steam, you need to buy the DLC there too. Sadly this isn't the old days where you could buy the game in store and the expansion disk in another...