You may find you have to move forward (just slightly) before this command is offered, after which you can change direction (forward/back) as much as you want.
Just take over the train manually and reverse it. Or if you are trying to back it up with programming, put a trackmark at the point that you wish it to move to and issue a 'navigate to' or 'drive to' command.
I use TRS2004, but I believe that the process is the same in later versions.
The process works in both Cab and DCC mode.
Click RMB on the loco. That should bring up the list of valid commands.
In this image you can see that there is a Drive command after the first RMB click, but no “Reverse”.
If I move just slightly forwards the next RMB click shows a slightly different command list. This includes Change Direction which will allow me to go in the opposite direction to the last movement, i.e. reverse.