I got a MacBook Pro mid 2012 with OS 10.15.7 and I want to upgrade to a Big Sur OS but this system don't allow me to do it any suggestion?

A mid 2012 MacBook Pro cannot be upgraded beyond macOS Catalina 10.15.7 -- see the following from Identify your MacBook Pro model - Apple Support

MacBook Pro (Retina, 15-inch, Mid 2012)

Model Identifier: MacBookPro10,1

Newest compatible operating system: macOS Catalina

MacBook Pro (15-inch, Mid 2012)

Model Identifier: MacBookPro9,1

Part Numbers: MD103xx/A, MD104xx/A

Newest compatible operating system: macOS Catalina