GTA 5 Activation Error on Steam – CD Key Shows as Already Used

I purchased GTA 5 on Steam, and despite having the invoice and proof of purchase, I keep encountering an activation error. When I enter the CD key, it says "this key has already been used." I have tried everything to fix this issue—reinstalling the game, verifying game files, and checking for updates—but nothing has worked.

I have paid for this game, and now I cannot even access it. This is unacceptable, and I need an immediate solution. Please provide support and either activate my game properly or issue a new CD key so I can play. I expect a prompt response and resolution to this issue.