What do you mean it's not working? There are different modes it can work in.
This as written in the docu:
By default Traccar requests geocoding service for every location message from device, which can be expensive.
https://www.traccar.org/reverse-geocoding/
Its no longer the defaupt mode that every location is requested, so the default mode got changed in Version 5.
Updated documentation.
Hi,
in the default.xml there is now a
<entry key='geocoder.ignorePositions'>true</entry>
So if they don't have this with false in their own config file the default reverse geocoding does not work any longer. Maybe you should change this in the documention of reverse geocoding or remove this line from default.xml