You can try to add the printer on macOS using the built-in “Generic Postscript PPD” if you haven't already.
If Xerox driver features are really needed, you could try an unsupported work-around in which you install the Xerox macOS Common Print Driver from a closely related product which is supported on Catalina. Note that there may be incompatibilities that could cause not all feature/function to work properly.
my Mac was upgraded to Osx Catalina and now my Xerox 6121 MFP doesn’t print.
I have the error code “Filter Failed”.
In the driver section I can find only the Osx Yosemite drivers, so, how can I resolve this problem?