It gives this error every time repositories refresh. Not only manually, for instance zypper dup does the same thing.
$ sudo zypper ref
[sudo] password for root:
Repository 'games' is up to date.
Repository 'Arc_and_Papirus (openSUSE_Leap_42.3)' is up to date.
Repository 'X11:Pantheon' is up to date.
Repository 'home:kistelini' is up to date.
Repository 'games' is up to date.
Repository 'home:etrash:arduino' is up to date.
Repository 'devel:languages:go' is up to date.
Repository 'home:dubkat' is up to date.
Repository 'utilities' is up to date.
Repository 'home:mgerstner' is up to date.
Repository 'devel:languages:go' is up to date.
Repository 'home:kill_it' is up to date.
Repository 'games:tools' is up to date.
Repository 'home:michals:arduino' is up to date.
Repository 'CrossToolchain:avr' is up to date.
Repository 'devel:languages:go' is up to date.
Repository 'multimedia:musescore2' is up to date.
File '/repodata/repomd.xml' not found on medium 'http://download.opensuse.org/repositories/openSUSE:/Tumbleweed/standard/'
**Abort, retry, ignore? [a/r/i/...? shows all options] (a): **i
**Do you want to disable the repository openSUSE:Tumbleweed permanently? [yes/no] (no): **no
Skipping repository 'openSUSE:Tumbleweed' because of the above error.
Repository 'home:michals:arduino:bin' is up to date.
Retrieving repository 'Packman Repository' metadata ....................................................................................................................................[done]
Building repository 'Packman Repository' cache .........................................................................................................................................[done]
Repository 'packman' is up to date.
Repository 'Packman Repository' is up to date.
Repository 'openSUSE-Tumbleweed-Non-Oss' is up to date.
Repository 'openSUSE-Tumbleweed-Oss' is up to date.
Repository 'openSUSE-Tumbleweed-Update' is up to date.
Repository 'skype (stable)' is up to date.
Some of the repositories have not been refreshed because of an error.
It is quicker to read other recent posts than to post and wait for someone to give you a personal answer. At least two other people have had this problem this weekend. It seems to be a result of using alien one-click installations… It is better to use a regular rather than a rolling distribution if you want to use alien packages.