"You Do Not Have The Required Permission" – as Root!!!

I'm trying to set up a new Admin account on my Mac mini i5 running High Sierra, for troubleshooting. I want to point iTunes in the new account to the iTunes Library, which is on an external disk. I option-launch iTunes and navigate to the iTunes Library. When I choose it, I get "You Do Not Have The Required Permission" (Admin. account, remember).

I then do a 'Get Info' on the iTunes Library disk. Under 'Permissions' 'System is shown as having 'Read & Write' (so that's admins, right?) I then try to add the current user under 'Permissions', so I can assign read/write permissions: "You Do Not Have The Required Permission".

Go back into the main Admin account, do 'Get Info' on the iTunes Library, try to add the new user account under 'Permissions': "You Do Not Have The Required Permission"

So, time for the big guns: I enable Root. Log in as Root, do a 'Get Info' on the iTunes Library, go to add the new user under 'Permissions': you've guessed it – "You Do Not Have The Required Permission".

I'm Root!!! How can I not have the required permissions?

Does anyone have any idea what's going on? Disk Utility First Aid run on the volume to repair permissions...

TIA