GPG Mail can't decrypt message
Sometimes things go wrong and a message is not decrypted as expected.
An error message about a missing secret key resulting in a message not being decrypted can occur under various circumstances. It indicated that the message author may have used the wrong public key to encrypt the message in question. As the required corresponding secret key does not exist on your system, you are then unable to decrypt.
This can happen when multiple keys for your email address exist and your contact may still be using one of your old keys which you have replaced a while ago. Another possibility is, that somehow your own secret key got lost on your local machine. This can happen, if you transition to a new computer and forget to transfer your secret keys. The solution would then be to recover your secret keys from a backup.
We recommend to upload and verify your current key on the key server.
Please double check with the message author that they are using the correct public key of yours. To do that you can verify and sign each others public keys.