Unable to change permissions on my internal hard drive Mac OS 10.13.6

I am having an issue with my internal hard drive. I wanted to save a file that I was working to and a message came up saying that I do not have permission to write to the drive. I "Got Info" and it said that I could Read & Write system but Read only for the wheel and for everybody. I unlocked the Finder and tried to change the Permissions but I get a message stating that: "The operation can’t be completed because you don’t have the necessary permission." This is my home/professional computer - I am the one and only Administrator. Any idea what could be going on? I'm running a 12-core mid-2012 Mac Pro with 80Gb memory.