I cannot remove a partition from my Mac

Do you have or know how to create a backup? You will need a spare drive on which to save the backup. You only need to backup your files - that is your entire Home folder - Desktop, Documents, Downloads, Library, Movies, Music, Pictures.

Once you have completed making a backup the remainder is pretty easy:

Install El Capitan or Later from Scratch

If possible, back up your files.

1. Restart the computer. Immediately, at or before the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

When the above is finished and you have re-booted the computer, and if all seems OK, you can restore your files by copying them from the backup to the corresponding folders on the computer.

Yes, the format type does matter. If you are formatting an SSD use APFS. If you are formatting a HDD, then use Mac OS Extended, journaled. Both use GUID as the partition scheme.

The backup can be whatever you prefer. If you already have a Time Machine backup then use it. See How to use Time Machine to back up or restore your Macs. The Time Machine backup drive must be formatted using Mac OS Extended, Journaled. Even if it is an SSD.

Did you use Boot Camp to create the partition? If so, then using Disk Utility to try removing it has altered the boot loader information rendering the partition unremovable. You need to backup your data then reformat the drive as a new single volume as it was before you tried partitioning it. Then reinstall macOS and restore your files. If you want a Windows partition you should use Boot Camp Assistant in the Utilities' folder. Don't allocate so much space to the Windows volume. Be sure to leave enough free space for macOS - 20GBs or 10% of the disk's capacity, whichever is greater.

Format the drive as Mac OS Extended (Journaled) with Option GUID. Time Machine can be used as the backup method.