You can try to manually put the key into your registry base.
To do so, hold together [Windows] + [R], input 'regedit', press [ENTER], from there, go to 'HKEY_CURRENT_USER\Software\MountAndBladeWarbandKeys'.
In there, right click on the right, and click on 'New' → 'String Value', rename what has been created for 'serial_key' or 'serial_key_nw' if it's a Napoleonic Wars key.
When you're done, double click it and input your key (make sure it's the right one), it is strongly advised to get the key you'll input here from your TaleWorlds account since you already registered them which means those are verified working keys.