I have installed the add-on "Aerosoft tool simple traffic" for ai traffic in FS 2020 simulator. Ai traffic works fine at airports and in the air. However, I can't get ai planes to appear on the cockpit screen of FBW's Airbus 32 nx. The white text "TFC" does not appear at the bottom of the ND screen?

What has that to do with which AI traffic you have installed? The TCAS doesn't care if there is traffic or not when you switch it on.

The message not displaying points more to the fact that you didn't actually switch it on. Make sure the Transponder is also in AUTO. By default it is off. Then you can switch the TCAS to TA/RA all you like, it won't work.