I am new to Cubase, but have been working with Logic for several years. I am on macOS/Monterey, and using EastWest Opus sample libraries. I am having no luck trying to add automation to tracks (Mod Wheel and Expression) via the sliders on my midi keyboard (M-Audio Oxygen Pro Mini). I can control the Mod Wheel and Expression while the track is playing, but Cubase does not record the automation. And, yes, I do have the Write button activated. I am able to manually write in automation.
Hello and welcome.
If your keyboard is just connected as a MIDI keyboard (and not as a remote device) it will send regular midi data to Cubase for Modulation and Expression. By default this can be recorded like you would record notes, ie. by recording them onto a track.
If you want to use VST automation instead of MIDI CC you need to set it up accordingly. Go to the MIDI menu and select “CC Automation Setup”. There you can define that either all incoming MIDI CCs should be converted to VST automation or just specific controllers.
This is a setup to only convert Modulation and Expression to VST automation:
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Wow, thanks so much for your help, Johnny! So easy–if you know what you’re doing. 
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