The error is due to ....
One of the Repos you have install is out of service temporarily or permanently. You may have gotten the wrong address Repo or Repo that has closed or is denying connection for other reasons.
solution:
It is a problem that needs solving external administrator Repo, try to reconnect after a few hours.
If the problem persists for a few days Repo definitely can eliminate to avoid the error message by going to Manage / Sources and clicking the Edit button