I installed Catalina again and now I can't solve the problem of file sharing in the "settings panel". The checkbox simply won't be checked and I can't add/share any disks or folders. It seems like they cut out SMB altogether.

But there is a solution:

Enabling Sharing on Catalina (OsX 10.15)

1.  Open Terminal
2.  Copy: sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.AppleFileServer.plist
3. Press [Enter]
4. Type your password (not displayed) and press [Enter]
5.  Copy: sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.smbd.plist
6. Press [Enter]
7. Type exit and press [Enter]

When the process is complete, close Terminal

Open system preferences and go to Sharing. That's it!