The operation can’t be completed because you don’t have the necessary permission again

I just got a PowerBook (mid-2012) with Catalina installed. It's my first experience with Catalina, and I'm having problems. What I want to do is simple, I think. The internal hard drive is named "tiny mac". I am the admin user. I want to have an alias for my Documents folder two places: in the dock, and directly under the hard drive (along with Applications, Library, etc. I can create the alias with no problem, but I can't copy it either place. When I Get Info for the drive I see that system has RW permissions, but admin read only. I am logged in as admin (it's the only account on the machine). When I try to change the permissions "I get the The operation can’t be completed because you don’t have the necessary permission" message.

I tried disabling csrutil from terminal. After an "operation successful" message I rebooted, but I still can't change admin to RW. Is this something Apple has made impossible? "csrutil disable" worked for me under earlier versions of OSX.