[HELP] Can't reclaim deleted Windows partition space into main hard drive, real frustrating.

I think you need to repartition the entire drive. I think this will do the trick and install macOS, as well.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

If possible backup your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

Note:

1. To install the version of OS X that was currently installed use Command-Option-R.

2. To install the original factory version when the computer was new use Command-Option-Shift-R.

The above requires you have a model that supports Internet Recovery. Preferably use Ethernet because it is faster and more reliable than Wi-Fi. If you are not sure whether your model qualifies then generally if you have a 2010 or earlier model then it does not qualify. Late 2011 models do qualify.

Post the output of

sudo gpt -vv -r show /dev/disk0

If you get a permission denied error, disable SIP - SIP - Enable and Disable steps and retry.

From macOS Terminal, post the output of

diskutil list

diskutil apfs list

Is Untitled a partition of Macintosh HD or a separate drive? I can only see its parameters. I would use Disk Utility to set the partition, Untitled, to use GUID for its partition scheme and format it APFS. Then you can remove all partitions and start over. The process I posted earlier should accomplish that.

It appears that the Delete [-] button is active when the above was made. What happens when you click on it to delete the partition, Untitled? Can you grab the circle gadget on the left and drag it up to the top?

In the bottom image above what is listed in Show Details? It should have the reason for the failure.

In the above note the device hierarchy.

ASMT 2115 = out-dented or topmost entry - physical disk.

Mojave Test = indented entry - logical volume.

This is what a hierarchy looks like in Disk Utility.