Lexmark hasn't upgraded their drivers for your printer for the Mac for 3 years. https://www.lexmark.com/en_us/printer/11134/Lexmark-CX517de
You are probably better off running Windows on your Mac, or getting another printer that is compatible with Big Sur. See also this tip on creating a second partition:
https://support.apple.com/guide/disk-utility/partition-a-physical-disk-dskutl14027/mac
And then running the internet restore to install an older operating system:
https://support.apple.com/en-gb/HT204904 - especially the part about command-option-shift-R restore.
You might find an open source solution that works, but many of those lose duplex and other setting*:
http://www.macmaps.com/printersx.html
*Links to my pages may give me compensation.