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.

1. Click the  symbol in the Menu bar.

2. Click Restart….

3. Hold down Command + R to boot into Recovery Mode.

4. Click Utilities.

5. Select Terminal.

6. Type csrutil disable. This will disable SIP.

7. Press Return or Enter on your keyboard.

8. Click the  symbol in the Menu bar.

9. Click Restart….

Extra Steps

1. Log in / boot up the Mac.

2. 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.

1. Hold down Command + R to reboot into Recovery Mode.

2. Click Utilities.

3. Select Terminal.

4. Type csrutil enable. This will enable SIP.

5. Press Return or Enter on your keyboard.

6. Click the  symbol in the Menu bar.

7. Click Restart….

Fastest way to fix this is the following.

1. Reboot
2. Hold Cmd + R
3. Choose language
4. If you are using an encrypted disk, select "disk utility" then select the disk where the unremovable files are located, and mount it. Once this has been mounted, quit disk utility.
5. In the Utilities menu select Terminal
6. Type cd .. as many times that are needed until you can see /Volumes
7. Type ls to see if you are where /Volumes is present
8. Type cd Volumes
9. Type ls to see name of harddrive
10. Type cd *nameofharddrive*/Applications
11. Type rm -r Install (press tab to write out the whole filename)
12. Press enter to delete
13. Reboot

I have found another solution in case rebooting in Recovery mode is not an option for you.

1. Move the offending Mac OS X installer back from Trashes to Applications

2. Re-download respective installer (e. g. the High Sierra using AppStore from https://support.apple.com/kb/HT208969) overwriting the „broken“ installer with a fresh one fixes the permission issue.

3. Drop the installer from Application to Trash. Empty trash will work now, deleting the installer correctly.