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 <start> and <size> 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