There is a definite problem with the setup structure for your startup volume. You should follow this guide:
Repair the Drive
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility and press the Continue button.
- Then select the indented (usually, Macintosh HD) volume entry from the side list.
- Click on the First Aid button in the toolbar and wait for the Done button to activate. Click on it, then quit Disk Utility.
- Select Restart from the Apple menu.
Backup your files to an external drive.
Install El Capitan or Later from Scratch
If possible, back up your files.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility from the Utility Menu and click on the Continue button.
- When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
- Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then wait for the Done button to activate and click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
This should install the last version of macOS that was installed on the computer.
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An alternative to the above is this:
Internet/Network Recovery of El Capitan or Later on a Clean Disk
If possible back up your files before proceeding.
- Restart the computer. Immediately after the chime hold down the (Command-Option-R) keys until a globe appears.
- The Utility Menu will appear in from 5-20 minutes. Be patient.
- Select Disk Utility and click on the Continue button.
- When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the side list.
- Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
- Set the partition scheme to GUID.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then click on the Done button when it activates.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
This should install the current or last version of macOS installed on the computer.