Can't Remove "Previous System"
It looks like you jumped ahead to some steps that aren’t needed. Apple recommends these steps:


Quit any programs you know were using the file, then try emptying the trash.
If that doesn't work, restart your computer to kill off background processes, then try emptying the trash.
If that doesn't work, restart your computer in safe mode by holding down the shift key as it boots to prevent login items and automatically launched programs from running, then try emptying the trash.
If that doesn't work, restart your computer in Recovery mode by holding down ⌘-R, and when it boots, select "Disk Utility". In Disk Utility, use the "First Aid" feature to repair the disk containing the file you wish to delete. Then reboot your computer and empty the trash.


That is the end of the advice in the linked article from Apple, but if all that fails, make a Time Machine backup, erase the drive and reload your data. If Disk Utility and Safe Mode don’t clear things, it’s usually more hassle for trained mac admin to diagnose and fix than erase and reinstall.