Check these
- You copied the key from the order page rather than typing it, with no leading or trailing space.
- The edition matches. A key for one edition or tier will not activate another.
- The app was installed from the vendor's own site, not a repack or a mirror.
- You are online, and no firewall or VPN is blocking the vendor's activation server.
- For account-based products: the email on the order is exactly the one your account uses.
If it still fails
Use Report a problem on the order, choose not working, and attach a screenshot of the exact error the vendor showed. Do not keep retrying — some vendors lock a key after repeated failed attempts.