I had the same problem and solved it in a strange way,
As we are all aware Mac user has actually 2 names: one is the username (the profile name if i may) and the Display name.
Prior to Catalina , when authenticating with an SMB share from a Windows computer , I was using the display name (although it is a full name with a space - strange for the traditional windows user...) and everything was cool.
In Catalina , I found out I can only use the username, windows cannot authenticate with the display name.
of course , before I got to this point I have checked/enabled SMBv2, created the NetBios file in the MAC and all the other hints I could find, which obviously did not do the trick for me.
hope this helps