The RTTOV GUI is not yet compatible with wxPython 4.1 so it is necessary to use python 3.7 and wxPython 4.0.4 installed with conda as described in the GUI documentation.
You could use this yaml file for conda and install this environment for RTTOV GUI with the command:
$ conda env create -f rttovguienv.yml
My colleagues are working on compatibility for wxPython 4.1.