i recently had something happen that might help. i turned off some windows 10 features i didn't think i needed. then mathcad 15 wouldn't start. so i tried turning them back on and mathcad 15 would start again. they are old .net installs. i'm not sure which one mathcad needs. but i turned them both back on to be safe. i attached a picture. the top two should be on. it might work with one of them off, but i'm not sure which one.
The installation requirements for Mathcad 15 are pretty clear:
Note that the .Net framework isn't a 'thing' that works with 'versions', where a higher numbered release includes the same supported functions as all previous releases.
.NET framework releases of different numbers coexist.
If an application requires .NET 3.5, it generally requires 3.5 and cannot work with 4.0 alone.
It's been many years since I installed Mathcad. I didn't remember what the requirements were. I'm not sure this is what the original posters issue was though. Just figured it was worth mentioning.