It seems you are hit by the already discussed issue, that the Nvidia drivers have a higher version as the open kernel module for Leap 15.6.
You can verify this by running zypper se -si nvidia in the terminal. If you see a version missmatch between the open driver kernel module and the rest of the drivers, you may switch to the closed source kernel module. This will solve the issue.
Your issue with the unavailibility of the Nvidia repo, may be caused by network unavailibility. You can append a 3 to the end of the kernel command line to boot with enabled network. See detailed description here.