How am I supposed to fix a disk from single user mode?
For Mojave and other recent os, recovery HD and disk utility are far superior for repairing the containers and volumes. 


https://support.apple.com/en-us/HT201314


Once you try twice to repair using disk utility, I might either look at the exact output from Disk Utility or then do the additional work repairing / deleting using the fsck family of tools, but I would use terminal from recovery instead of single user mode. 

Basically, you aren’t supposed to use single user mode for this anymore from what I can tell. Also consider, some errors Disk Utility will not fix. You are expected to back up your files and erase the disk at that point, reinstalling the OS and restoring from a backup.