I've tried using the Y2O3.xye demo file from your tutorial video and I get the same error message there.
When I launch Topas and click "Load Input files", then I get the following error message:
*** Error loading sstring_in
at LINE 21
See log file C:\Topas-7\topas.log
File extension not supported!
I don't even get to load the .inp file.
I've tried to look up the C:\Topas-7\topas.log file and this file is also missing in the folder.