buying a game for a friend

Provided the Steam gifting restrictions don't apply (price of game in your friend's region below 110% of your region) you should be able to send a game as soon as your invite was accepted.

could you pleas copy.paste the error?

click on market, to see if you have some limitation, there will be a message
if you select the game, select your friend, and go further, and have a message at the end, copy paste that, to see what's could be.

Did your friend make any purchases on Steam yet? They have to spend at least $5 on the Steam store before they are allowed to receive gifts and access some community features (local currency may be different, this is for the US). They can either purchase via a credit/debit card or purchase a Steam Gift Card from their local retailer if they are available in their country.