If this Nikon Z50 is a new camera to market Apple will only
make it compatible with its current macOS which is Big Sur.
Apple does not allow the Camera RAW database of older OS's
to be updated for new cameras.
Just because Nikon cameras all share the .nef extension for their RAW files does not mean
that they are the same, each camera writes the RAW file slightly differently and therefore
when a new model like the Z50 is released, the information for its RAW files has to be given
to Adobe, Apple, Affinity and any other RAW file processors so they can update their RAW
file database so these new files can be read.
This also goes for cameras from other manufacturers, Canon, Olympus, Leica etc etc etc.
If it is important for you to read the Nikon Z50 files on Catalina then download and install
the free Adobe DNG Converter. This converts the proprietary RAW file from a camera to the
.dng file format (Digital Negative), these files will be able to be read by your system.
https://supportdownloads.adobe.com/detail.jsp?ftpID=7053
Adobe tends to add new cameras to its RAW database far quicker than Apple.