mac os could not install

Try setting up another admin user account to see if the same problem continues. Also try the Safe Mode.

Safe Mode - About El Capitan
also Sierra

Safe Mode Startup – El Capitan. also Sierra

If that doesn't work, try downloading in a new admin user account.

shut down your Mac and wait 10 seconds, then press the power button to turn it on and immediately hold down Command + R and hold those until a little after the apple logo appears. It should take you to a choose language screen, choose and then you'll be at macOS utilities. Go to the wifi button in the top right and make sure your connected to the internet and then press reinstall latest version of macOS, click continue, agree and it should download and then restart, and install on the apple logo screen for about a half hour and then it should restart itself to the login screen. hope this helps!

Thank you Eric!

Update: Your link doesn't work anymore. For safemode-instructions go here Use safe mode to isolate issues with your Mac - Apple Support

You are welcome.

You are welcome. Both links work for me, but you posted an even better link. Thank you.