Please install Clip Studio Paint version 1.9.4 and above for macOS Catalina.
For information on compatibility with Sidecar on macOS Catalina, see the following article:
We are aware of the following problems with macOS Catalina.
• Clip Studio Paint versions 1.8.0 or prior are unable to be installed.
• In Clip Studio Paint versions 1.9.3 and prior, sometimes characters disappear when editing or exporting layers of text.