Could you try downloading Mathcad Express from this link? - https://www.mathcad.com/en/try-and-buy/mathcad-express-free-download. See if it helps.
Did you follow the installation rules as set out here:
"execute/run pim_installmgr_mathcad.exe, run as an Administrator."
Apparently, Prime installation will not run on some machines, see here: