After typing the command sudo apt update I get reply:

Err:1 http://pl.archive.ubuntu.com/ubuntu bionic InRelease

  Temporary failure resolving 'pl.archive.ubuntu.com'

Err:2 http://pl.archive.ubuntu.com/ubuntu bionic-updates InRelease

  Temporary failure resolving 'pl.archive.ubuntu.com'

Err:3 http://pl.archive.ubuntu.com/ubuntu bionic-backports InRelease

  Temporary failure resolving 'pl.archive.ubuntu.com'

Err:4 http://pl.archive.ubuntu.com/ubuntu bionic-security InRelease

  Temporary failure resolving 'pl.archive.ubuntu.com'

Reading package lists... Done

Building dependency tree

Reading state information... Done

All packages are up to date.

W: Failed to fetch http://pl.archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Temporary failure resolving 'pl.archive.ubuntu.com'

W: Failed to fetch http://pl.archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Temporary failure resolving 'pl.archive.ubuntu.com'

W: Failed to fetch http://pl.archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Temporary failure resolving 'pl.archive.ubuntu.com'

W: Failed to fetch http://pl.archive.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Temporary failure resolving 'pl.archive.ubuntu.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

and after typing the command sudo apt install snmp snmp-mibs-downloader I get reply:

Reading package lists... Done

Building dependency tree

Reading state information... Done

Package snmp-mibs-downloader is not available, but is referred to by another package.

This may mean that the package is missing, has been obsoleted, or

is only available from another source



E: Package 'snmp-mibs-downloader' has no installation candidate

What are these errors and how can I fix them?