1. The key is for a different edition
This is the most common cause. A Home key won't activate Pro. Check your edition in Settings › System › About and compare it with the key you bought. If you run Pro and have a Home key, you need a Pro key instead.
2. A character was mistyped
Letters and numbers that look alike are easy to confuse: B and 8, O and 0, S and 5, G and 6. Copy the key directly from your purchase confirmation and paste it with Ctrl + V.
3. The PC is offline or the clock is wrong
Activation needs an internet connection and an accurate clock. Under Settings › Time & language › Date & time, turn on Set time automatically and select Sync now. A wrong date can cause secure-connection errors such as 0x80072F8F.
4. Windows is in S mode
Home in S mode must be switched out of S mode (free, via Microsoft Store) before changing editions.
5. The Settings dialog fails but the key is fine
Try the command line. In Terminal (Admin) run slmgr /ipk YOUR-KEY and then slmgr /ato. The message that appears often contains a more specific error code.
6. The key was used on another PC
A retail key activates one PC at a time. If you moved it from an older PC, see activation after hardware changes. An OEM key can't be moved to a different PC.
7. You recently changed the motherboard
Windows treats a new motherboard as a new PC. A retail license can generally be reactivated, subject to Microsoft's licensing and activation rules: try the Activation troubleshooter, or on supported builds phone activation (slui 4).
Still not working?
Write down the exact error code, then look it up in Windows 11 activation errors. Then contact the seller you bought the key from with your order number, your Windows edition and the error code. Replacements and refunds for keys that can't be activated are handled by the seller under its own policy.