Can't remove "Install macOS High Sierra.app"
I was experiencing this problem after downloading High Sierra to fix a friend's Mac. I found Rick's solution worked for me, but I've added a few more steps.


Click the  symbol in the Menu bar.
Click Restart….
Hold down Command + R to boot into Recovery Mode.
Click Utilities.
Select Terminal.
Type csrutil disable. This will disable SIP.
Press Return or Enter on your keyboard.
Click the  symbol in the Menu bar.
Click Restart….


Extra Steps


Log in / boot up the Mac.
Empty Trash.


I also found that somehow Previous System folder appeared under the Mac HD. So I also trashed that (which wouldn't trash previously).

I then rebooted the Mac a few times to check that everything was working fine. Then rebooted into recovery mode.


Hold down Command + R to reboot into Recovery Mode.
Click Utilities.
Select Terminal.
Type csrutil enable. This will enable SIP.
Press Return or Enter on your keyboard.
Click the  symbol in the Menu bar.
Click Restart….