Hello June.
It looks like Digital Transport For Africa made a copy of my GitHub repo a long time ago. I don't recommend using anything you find there.
First of all, this is the authoritative version of the Add GTFS to a Network Dataset toolbox is here: http://esri.github.io/public-transit-tools/AddGTFStoaNetworkDataset.html.
Note that this toolbox is now deprecated anyway, even if you get the latest version. I no longer recommend that people use this, and I do not provide extensive tech support anymore for it.
Second, that toolbox does not work in ArcGIS Pro. If you want to do analysis with public transit data in ArcGIS Pro, you do not need any additional, downloadable tools. The functionality is now available out of the box. Learn more here: https://pro.arcgis.com/en/pro-app/latest/help/analysis/networks/network-analysis-with-public-transit...
The best way to get started is to follow this tutorial: https://pro.arcgis.com/en/pro-app/latest/help/analysis/networks/create-and-use-a-network-dataset-wit...
Note that the tutorial includes some data and a network dataset template as well, but it's designed specifically to work for ArcGIS Pro.
Hope this clarifies things!