I happen to have the same issue:
N: Skipping acquire of configured file 'all/binary-amd64/Packages' as repository 'Index of /pool/WPB_18.0.51_64 all InRelease' doesn't support architecture 'amd64'
Ubuntu 20.04.6 LTS
Product version: Plesk Obsidian 18.0.53.2
OS version: Ubuntu 20.04 x86_64...