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Have you tried without putting the key prior to creating the giveaway ? Just suggesting a random change to see if it helps. I don't see why it would be a problem but, who knows. Otherwise, clearing cache and cookies might help.

Humble links need to contain "?gift" in the URL to be accepted.
To make a giveaway with your link, you'll need to select gift and give the link to the winner via Steam chat or mail.

I'm guessing you have a second line with a space or period or something else easily overlooked. The error sounds like a second invalid key.

Humble Bundle gives links in different formats, but I think only one is accepted by Steamgifts. You can try changing "downloads" for "gift" (https://www.humblebundle.com/gift?key=) and see if it works. I did it once without any problems when I had a similar situation.

And if it doesn't work, you can always give the link directly to the winner via Steam or e-mail.

As @TempeteJoachim said, be sure to use the ?gift format. Be very careful giving links in the downloads? format, as they're grounds to have your Humble Bundle account revoked.