This appears to be a system configuration problem. The file /mnt/repodata/repmod.xml is either not available, not readable, or not writeable. In any case, normally, it would not be in /mnt, so I expect that you did something to alter the configuration of yum that caused this.
Since I don't find anything like this on my RHEL systems, I suspect that you may want to create the directory /mnt and try again. FWIW, what version of RHEL are you running?