The server is failed because it can't find "mos.local." How do I fix it?

You're going to need to give us some more context on the error message and on the particular tool that's reporting the message.

mos.local is probably a DNS host name.

Is this Server Admin that's reporting this?

Can you log in on the server console?

Connections (to a server or otherwise) can fail if the host name can't be translated to a network address, or if the host can't be reached.

If you have access to the server, log in and launch Terminal.app from Applications > Utilities folder and then issue the (non-destructive) command:

sudo changeip -checkhostname

it'll prompt you for an administrative password (and might print a caveat about using sudo, if you haven't used it before), and the changeip command will then print some information about the host and its name and an indication that the network and the host names are correct and no changes will be needed, or that there are errors.