apparently I can't upgrade to macOS Big Sur because I have a macOS Sierra Mid 2011 version

I just realized I need to update my computer to the very latest version for my iMac in order to continue with my online business.

I have checked for updates in the past and it says there are none. I did not realize I won't get updates because my computer is so old. Of course...I can not afford a new desk top at this time.

Does anyone know how I can skirt around this issue and get the latest version without having to purchase another one?

My on-line business is my only source of extra income even though it has been quite slow this past several months.

Thank you for any help

La