Not enough free space to even delete.

You have allowed the drive to become filled to the point where there is no free space left resulting in the drive becoming corrupted. You cannot put files in the Trash nor Empty the Trash. You cannot copy and paste or move files. The disk directory is corrupted beyond repair. You cannot back up either. You must do this:

Install El Capitan or Later from Scratch

     If possible backup your files.

1. Restart the computer. Immediately after 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 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 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.

This will install the version of OS X you had installed.