The operation can’t be completed because you don’t have permission to access some of the items

What file do you want to reclaim? You need to turn off the SIP, first.

What is SIP and when is it safe to turn it off?

Disable System Integrity Protection (SIP) - Rootless In OS X El Capitan - How To

Turning Off Rootless System Integrity Protection in OS X El Capitan 10.11.x or Later

Again, the vast majority of Mac users should not disable rootless. Disabling rootless is aimed exclusively at advanced Mac users. Do so at your own risk, this is not specifically recommended.

Reboot the Mac and hold down Command + R keys after the startup chime, this will boot OS X into Recovery HD. When the Utility Menu appears, select the Terminal from the Utilities menu in the menubar. Paste the following command into the terminal then hit return: csrutil disable; reboot.

You’ll see a message saying that System Integrity Protection has been disabled, and the Mac needs to restart for changes to take effect. The Mac will then reboot itself automatically. You can also issue the command by itself without the automatic reboot like so: csrutil disable.

Once the Mac boots up again, System Integrity Protection will be disabled entirely in OS X.

Checking the Status of Rootless / System Integrity Protection in OS X

If you want to know the status of rootless before rebooting or without rebooting the Mac from the Recovery HD, just paste the following command into the Terminal: csrutil status

You’ll either see one of two messages:

$ csrutil status

$ System Integrity Protection status: enabled

or

$ csrutil status

$ System Integrity Protection status: disabled

If at any time you want to change the status of rootless, another reboot from the Recovery HD is required.

How to Re-Enable Rootless System Integrity Protection in OS X

Simply reboot the Mac again from the Recovery HD as directed above, but at the command line use the following syntax instead: csrutil enable

Just as before, a reboot of the Mac is required for changes to take effect.

The SIP prevents deletion of all sorts of files whether system or not. Only files in your Home folder can be removed by you except for some in the invisible Library folder. If you think it is a permissions problem, then select the file, press COMMAND-I to open the Get Info window. At the bottom, click on the arrowhead by Sharing & Permissions to open the panel. You can then change permissions by first clicking on the lock icon and entering your admin password. You may find other permission problems in your Home folder so here is a way to fix all of them: RepairHomePermissions 1.2 (Sierra, High Sierra and Mojave.) It will also work in Catalina.

Note that files from your computer are not going to be writeable or editable on someone else's computer unless you set the "everyone" user to Read & Write. This may not be desirable for security issues, so use the Shared folder locate in the Users folder - :HD_Drive/Users/Shared/.