Is it possible to recover "Could not find APFS system volume handle!" which is reported in verbose mode?
The best chance for data backup (assuming you don’t have any backups) is to connect the Mac to another in target disk mode. 

If you don’t want to pay for recovery software, you could boot to recovery HD and let disk utility try to repair the volume. This can cause further data loss (the file system heals but files with data get deleted). 

Disk Utility isn’t designed to lose data and most of the time will warn you if the next step is expensive data recovery or an erase and reinstall.