No Administrative rights in Catalina

Reinstalled my High Sierra from USB after formatting internal drive. Then upgraded to Catalina, the high mark as far as I can tell for my iMac 21.5-inch, Late 2013 with Intel IRIS Pro 1536 MB graphics card. (Anybody know how to bump it up?)

I am the only user on the machine.

I am trying to format through Terminal commands a USB drive that doesn't even show in Dick Utility. But Terminal gives me "command not found"; or "sorry" (really...). especially if I have done: %su username then enter and put in the password.

I have used Macs for years and never, ever encountered this problem. Sigh, another bridge to cross. Any help out there? Many thanks.