Well, since you've posted this Q in the Catalina community, I assume you are using macOS 10.15.x in which case iTunes is no longer a thing. It has been replaced by the Music app, and that app is no longer the way one would sync or manage an iPod.
Being that you're trying to synch an iPod Shuffle, I suspect the device is no longer compatible with the newer macOS, though you didn't mention which shuffle version you have.
Ensure that your Catalina OS has been updated to it's most recent version 10.15.7.
Then please have a look at this admittedly older thread on a similar subject:
iPos shuffle and Mac Catalina - Apple Community