Cannot refresh snap-declarations for snaps on airgapped proxy

while doing “sudo snap refresh snapd” on Ubuntu core device which is configured to refresh snaps from on-prem snap-store-proxy( which is in air-gapped mode) it shows below response.

error: cannot refresh "snapd": cannot refresh snap-declarations for snaps:

       - curl: account (trElzADL6BSHUJX2R38cUoXIElh2BYRZ) not found
       - intel-iotg-kernel: snap-declaration (v7evc8smSqgPwL0KCjpGJf3KuucG2Yap; series:16) not found
       - pc: snap-declaration (UqFziVZDHLSyO3TqSWgNBoAdHbLI4dAH; series:16) not found

why does this shows this?