You can download older versions of external tools by clicking on the "View All" link on the tool's page:
After you have downloaded the .zip file for a particular version of the tool, you can add it to Skyline using the menu item:
Tools > External Tools
and then press "Add > From File".
I hope you are able to get the older version of MSstats to install successfully. People seemed to be having trouble getting it to install because some of the packages for R version 3.6 did not seem to be where they were supposed to on the internet anymore. For instance, this person was having trouble with a package called "tibble".