The problem lies in fhRAR plugin itself. If it is installed TC handles RAR archives using it. This plugin doesn't support adding files to archives and because of that you've got an error.
The only solution is to disable this plugin permanently and use TC's engine (with WinRAR enabled) for RAR handling BUT it will be not possible to extract multi-volume archives easily. You will have to use UniversalExtractor utility (included in TC UP) or mouse context menu WinRAR option.
If you want to switch to WinRAR after all, please follow the instructions below:
1. open wincmd.ini file in text editor,
2. find [PackerPlugins] section and delete line with:
Code:
rar=196,%COMMANDER_PATH%\PLUGINS\wcx\fhRAR\fhRAR.wcx
(It should be line number 361.)
3. save the file and restart TC UP,
4. go to "Configuration" -> Packer
5. on RAR group specify your WinRAR executable path,
6. you can select "Use internal module..." option if you'd like to,
7. Apply and OK should do the rest.