Why are you using a 7835 driver on a 6015?
As you can see from our Compatibility Matrix Xerox will not be releasing Catalina drivers for the WC 6015. However, you can still add the printer on macOS using the built-in “Generic Postscript PPD”.
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
For the 7835, here is where you can download the Catalina driver.
I will update my MacOS 10.15 Catalina two days back, after updating printer is not supporting