The moment you enter trade URL, we are checking if your Steam account can accept trade offers. That's done to make sure we can send you the skin you've earned.
If you see an error saying "The trade link is invalid", it means there's something wrong with your Steam account. It's not opened for trade, so we couldn't have established a connection. Please check those:
That's a button above. Click it to check.
Or follow that link to check the status of your Steam Guard.
If it's not enabled, Steam may block your account from receiving trade offers.You need to have your inventory settings set to Public to accept trade offers from us.
Or follow that link to check why you can't trade.
You need to be logged in to your Steam account on your device, so Steam could check your account and explain why you are not able to accept trade offers.