Your issue is right there in the output:
Apt can’t verify the certificate chain in your Nexus install. The reasons for this can be many (none anything to do with Nexus per-se). Since you’re using https to get to your repository, my guess is that the TLS cert can’t be verified. My guesses as to why are:
- You use a self-signed cert, so will need to put the public bit of the root CA cert into the Ubuntu cert store
- You use a public cert, but haven’t put the full chain into the TLS config on your nexus server.
My knowledge is less certain, but it could also be that you need to add the public bit of the repository signing key to the Apt trust store.