Reprogram it with the right username and password.
Nobody can supply more than that because you supplied nothing to work with.
Scan to mailbox?
Scan to SMB?
Scan to FTP?
Scan to Home?
Scan to Email?
Did it work and stop?
First time setup?
A document was scan to a network scan directory, but we get this message - Login Failure. check user password, and or setup.
The scan has worked before, but once in a while the scanning will not work.
Assuming current levels of firmware, a login failure is a login failure.
There was at one time an issue with Server 2012 using SMB where scans could work or fail dependant on pagecount, It only happened with server 2012 and with the release firmware that ended with 34540.
Beyond that I don't believe anyone can suggest anything since we have no idea on your setup, even now you haven't explained if it is SMB,FTP or either of those wrapped in Scan to home which would introduce network logins to the equation as well.