Normally Prime should be in the list of applications (ether the top list, or the list under "Other options". If that's not the case you should find at the bottom the link "Look for another app on this PC" which gives you the opportunity to point to a specific .exe file on your computer. If that is wat resulted in finding the MathcadPrime.exe in the folder you showed, then you did the right thing.
Apparently there's something not right on your PC.
Did you remove the first Prime neatly, according Windows rules, or did you just clear or delete the directory where it was installed?
And did the re-install run without strange phenomena?
Maybe you have to contact PTC support to help you. Note that this forum is NOT PTC support.
Success!
Luc