500MB missing from 1TB SSD after adding partition failed

I tried to add a partition in disk utility but it failed - I don't remember the error or message - but I have yet to be able to recapture that space.

Originally, it was High Sierra on my mid 2012 Mac Book Pro. I did a clean install of Sierra, upgraded to High Sierra, migrated only my user files with time machine then tried to add a Mac Extended Journaled partition which failed, so I tried again. The pie chart in disk utility showed the original High Sierra space (now APFS) which had been reduced to 500GB, plus 2 Mac OS Extended (Journaled) partitions, at least one of which was not mounted. I went around in circles with chat gpt with disk utility, terminal and recovery mode.

Since then, I booted with Sierra on a USB, planning to erase and reinstall but it didn't work. In internet recovery mode, I'm now showing a 500 MB SATA Internal Physical Volume - Mac OS Extended (Journaled) plus a 2GB MacOS Base System and that's it.

I would appreciate any suggestions. I checked the drive with first aid prior to this issue.

Update: the pie chart for the SSD shows 999 GB free space. It does seem to give me the option to resize the partition to 1000 GB...but this fails. It says that Volume header needs minor repair, the partition needs to be repaired, File system check Exit code is 8 and : (-69803). However, first aid isn't showing any problem in Internet Recovery mode.