A: No. Steam refund policy applies only to direct Steam purchases. Keys are non-refundable via Valve. You must ask the seller.
A: This often means the key requires a different Steam account region or is a beta/test key that expired.
Once logged in, tap the hamburger menu (three lines) or your profile avatar → look for “Activate a Product on Steam” (sometimes under “Account” or “Store” submenu).
However, there’s a catch: like the desktop client. Instead, you must use the mobile web browser or a hidden workaround within the app.