Signature verification failed for file 'repomd.xml' from repository 'openSUSE-Leap-15.1-Update'.

As per the subject line really.

Signature verification failed for file 'repomd.xml' from repository 'openSUSE-Leap-15.1-Update'.

    Note: Signing data enables the recipient to verify that no modifications occurred after the data
    were signed. Accepting data with no, wrong or unknown signature can lead to a corrupted system
    and in extreme cases even to a system compromise.


    Note: File 'repomd.xml' is the repositories master index file. It ensures the integrity of the
    whole repo.


    Warning: This file was modified after it has been signed. This may have been a malicious change,
    so it might not be trustworthy anymore! You should not continue unless you know it's safe.


**Signature verification failed for file 'repomd.xml' from repository 'openSUSE-Leap-15.1-Update'. Continue? [yes/no] (no): **no
Retrieving repository 'openSUSE-Leap-15.1-Update' metadata ....................................................................................................[error]
Repository 'openSUSE-Leap-15.1-Update' is invalid.
[repo-update|http://download.opensuse.org/update/leap/15.1/oss/] Valid metadata not found at specified URL
History:
 - Signature verification failed for repomd.xml
 - Can't provide /repodata/repomd.xml


Please check if the URIs defined for this repository are pointing to a valid repository.
Warning: Skipping repository 'openSUSE-Leap-15.1-Update' because of the above error.
Some of the repositories have not been refreshed because of an error.
Loading repository data...
Reading installed packages...


Nothing to do.

Did you try it some times?
Maybe the Server is updating.
Or broken, because download.opensuse.org is a redirector which switches to a mirror near by you.