Disk disappears while trying to upgrade to Mojave, error ppm002
Error PPM002 points to a memory (RAM) issue. See Apple Diagnostics: Reference codes for more info.

Edit #1: I'll expand a bit - I had a 2014-era iMac in for repair early last year because it wouldn't update to High Sierra from a seemingly working Sierra, and in the end it turned out to be faulty memory. Whether this was due to a specific way that macOS accessed the RAM during an install, I don't know. What I do know is that it was faulty RAM and replacing it solved the problem. There was a PPM error, though I don't recall the exact number.

Your MacBook Pro (assuming Mid 2017 model) has soldered RAM, which means if it is faulty, then it's going to involve an Apple repair or replacement. Unfortunately it's not possible to just pop the RAM out and replace it.

Some things to try:


run Apple Hardware Test to do a full check of your RAM. Any Apple authorised technician will run this as part of their tests, so it's useful to get something concrete.
use another Mac to make a bootable High Sierra (not Mojave) USB stick, take a full backup of the drive using Disk Utility. The bootable installer (which doesn't actually require you install anything to the hard drive) is more lightweight on RAM use, so you have a good chance of getting your data backed up. If you're using Time Machine and it's up-to-date, disconnect that for now so you know you have a relatively safe backup if your MacBook does need to be repaired/replaced.