Any ideas how to solve this issue?
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A8580BDC82D3DC6C
Get:10 http://gb.archive.ubuntu.com/ubuntu focal-updates/main amd64 DEP-11 Metadata [196 kB]
Get:11 http://gb.archive.ubuntu.com/ubuntu focal-updates/main amd64 c-n-f Metadata [8,092 B]
Get:12 http://gb.archive.ubuntu.com/ubuntu focal-updates/universe amd64 DEP-11 Metadata [177 kB]
Get:13 http://gb.archive.ubuntu.com/ubuntu focal-updates/multiverse amd64 DEP-11 Metadata [2,468 B]
Get:14 http://gb.archive.ubuntu.com/ubuntu focal-backports/universe amd64 DEP-11 Metadata [1,972 B]
Get:15 http://security.ubuntu.com/ubuntu focal-security/main amd64 DEP-11 Metadata [21.2 kB]
Get:16 http://security.ubuntu.com/ubuntu focal-security/universe amd64 DEP-11 Metadata [35.8 kB]