Last week the signing key for the CubeCoders repo was replaced with a new one (we currently cycle them every 2 years). This means that without the new key you will not be able to update AMP.
To get the new key, simply run the following as root:
apt-key adv --fetch-keys https://repo.cubecoders.com/archive.key
You can then run
apt update and
apt upgrade as normal.
This was previously announced on Twitter and Discord last week, copying here for anyone who didn't see those or who disabled their news feed in the panel.