Been trying to get the iMac to connect with a windowsNT 4 pc. The iMac shows up in network neighborhood, but when I double click on the iMac get an error message - "\\imac is not accessible.  The network path was not found.

From the windowsNT PC, I can successfully ping the iMac by name and IP address, and can ping from the PC from the iMac. This is how the iMac sharing is configured:

- have a public folder set as the shared folder, with everyone identified as having read and write privileges
- under sharing/options,"share files and folders using SMB (Windows) is selected
- below that, have the "on" button checked for the accounts I have on the iMac.

I have tried to connect to systems running Mac OS Sierra, High Sierra, 2 x Mojave and Catalina

I’m not entirely sure but it could be the difference in SMB versions. Although you could try setting a shared folder on NT4 and then connecting from the Mac to the new shared folder. 

Do not create duplicate topics!

Maybe run an ftp server on one and use an ftp client on the other to move files.