Thank you.
There appears to be a problem installing the correct key. The problem needs to be investigated further.
If you need spotify, the key can be downloaded and added to your system.
(see
https://www.spotify.com/uk/download/linux/ for the source information)
If you wish to go ahead, open a terminal by pressing 'Ctrl' 'Alt' and 'T' together.
Type or paste in the command in the blue code box below followed by enter.
Enter your password if asked.
curl -sS https://download.spotify.com/debian/pubkey_6224F9941A8AA6D1.gpg | sudo gpg --dearmor --yes -o /etc/apt/trusted.gpg.d/spotify.gpg
This should allow the update/upgrade process to run.
If you don't want to do this, let us know and I will try to add instructions on how to proceed without spotify.