SAVED FOR FUTURE REFERENCE

https://apple.stackexchange.com/a/359056/457053

The Disk Utility application probably can not be use to fix your Mac. Below is a alternate procedure.

Start up the Mac from macOS Recovery over the Internet by restarting and immediately pressing and holding the the key combination Option-⌘-R.

From the menu bar, select to open a Terminal application window.

• Enter the following command to get the GUID Partition Table (GPT) entries.

$gpt -r show disk0

An example output is shown below.

$gpt -r show disk0
          start       size  index  contents
              0          1         PMBR
              1          1         Pri GPT header
              2         32         Pri GPT table
             34          6         
             40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
         409640   97656248      2  GPT part - FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF
       98065888        544         
       98066432   97245184      3  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
      195311616        891         
      195312507         32         Sec GPT table
      195312539          1         Sec GPT header

• Enter the command given below to change the GPT.

    gpt -f remove -i 3 disk0
    gpt -f remove -i 2 disk0
    gpt -f add -i 2 -b  <start> -s <size> -t apfs disk0

Where and are integers taken from the row with index equal to 2 in the table shown in step 2. An example is shown below.

    $ gpt -f remove -i 3 disk0
    disk0s3 removed

    $ gpt -f remove -i 2 disk0
    disk0s2 removed

    $ gpt -f add -i 2 -b  409640 -s 97656248 -t apfs disk0  
    disk0s2 added

• You will need to replace 409640 and 97656248 with the values shown in your GPT table.

• Restart the Mac back to macOS Mojave.

• Enter the command given below in a Terminal application window to reclaim the free space.

$ sudo diskutil apfs resizeContainer disk0s2 0