i got the same problem, but i do these steps:
1. cmd+R go to safe mode
2. go to safari by clicking get help online
3. go to this forum, and copy the commands on JohnsonYi (Dont type the commands, just copy the commands through safari)
diskutil apfs updatePreboot disk1s1 -od /Volumes/Macintosh\ HD/Recovered\ Items/private/var/db/dslocal/nodes/Default
dont forget to change ur disk
4. quit safari
5. go to terminal
6.Issue commands "diskutil apfs list" to find out the APFS volume disk name like "disk1s1".
For Macbook pro, the mount point should be "
/Volumes/Macintosh HD
"
For Macbook air, the mount point should be "/Volumes/air"
Let's suppose the APFS volume disk is "disk1s1", and you use Macbook pro(the mount point is "
/Volumes/Macintosh HD
").
7. Issue the following commands to unlock the disk
diskutil apfs unlockvolume disk1s1
8. Issue the following commands to updatePreboot for this disk
diskutil apfs updatePreboot disk1s1
9. paste the commands that u copy before from safari and dont forget to change the APFS volume disk
diskutil apfs updatePreboot disk1s1 -od /Volumes/Macintosh\ HD/Recovered\ Items/private/var/db/dslocal/nodes/Default
this is how solved error -69836, and reboot my mac. PROBLEM SOLVED!!!!!! hope these steps could help!!😊😉😎