macOS Mojave won't install updates past 10.14.1 due to requirement for APFS formatting
I had the same problem. After multiple searches, the only way I found to get the drive updated to APFS was the following:


Full clone of internal drive to external drive using SuperDuper! (Carbon Copy Cloner would also work from what I've read)
Reboot from external drive 
Reformat internal drive as APFS  
Full clone of external drive to internal drive using SuperDuper! 
Reboot from internal drive


It feels like there should be an easier way to do this, but I couldn't find it.