You apparently did not add a key for which you have the necessary private key. If you are in a corporate setting, you might not have your private key. If you do have the private key for the public key you added as a Master Key, right click on it in All Keys of PGP Destkop, select Properties, and set Trust as Implicit. If you can't do this, you don't have the private key. If this is the case, you can generate a new key that you use only for this purpose - just generate any key with any User Id you want, and after doing so, add that key to your Master Key List. This will only work on files that you encrypted after you added your key to the Master Key List.