Attempting to "trade" the gift?

Maybe you are banned from trading? Did you try trading with some one else?

maybe you have recently changed from a computer to another?

Do you have him blocked?

Common issue on Steam. Probably you, him or both recently activated Steam Guard (or one of you have it disabled), or are login from a different computer, or changed the password in the recent days. The only thing both of you can do for now is for him to send the gift directly through Steam to you or to your e-mail.

I have had that problem myself and sometimes using just the web browser it will work or if I shut down and restart steam it lets me until it does... steam is odd like that

Could always send them a message through steam.

Log out and in?

Every time i log in through browser I get error when I try to trade on Steam client. It works ok after logging out and in.

Here's what I did when I faced a similar problem.

1) Logout

2) Delete all cookies

3) Login

4) You should be able to trade now.

There is an option to send gift through email, it'll work. I couldn't gift one of my latest gifts to the winner, had to send to him via email from my inventory :)

Can't they just send the gift directly through Steam? I've had to do that with a few of my giveaways due to issues with trading.

He can also send it directly to you (without trading) like the old method (sending a gift to a friend).

Not sure what causes this when it happens randomly but I read once that changing the "Custom URL" setting in your profile settings had fixed it for some people in the past. Or maybe it was a different problem with the inventory. I honestly don't remember.