FACE A SYSTEM PROBLEM

Backup your files.

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 and progress bar appear. Wait until 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 target drive (out-dented entry w/type and size info) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the partition scheme to GUID.
6. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.

Restore your files from your backup.

Don't install any anti-virus or cleaning programs on your Mac as they are not needed and will cause more problems than they solve plus they impact system performance. Uninstall those apps by following the developer's instructions.

NEVER perform an OS upgrade (that is from 10.14 to 10.15) unless you have the time to deal with it failing or breaking something. Besides possibly breaking some older apps it is possible an old hard drive could be pushed over the edge as an OS upgrade is very stressful on an old drive that may already be teetering on the edge of failing.

Also ALWAYS have regular system backups so you can quickly restore your system to a working state if something goes wrong or you accidentally delete an important file. This is even more important when you use an SSD.