If you get this message after installing Mathcad Prime 12:
Then you need to install .NET 10.
Clicking on that 'i' will show this message:
And when you click on the link you will be sent to a Mathcad help page on ptc.com:
https://support.ptc.com/help/mathcad/r12.0/en/#page/PTC_Mathcad_Help/wn_net10.html
Click on the link and you will get to this page:
Clicking on the Download link will take you to the Microsoft download page for .NET 10. The specific version will change over time. What you need is the x64 installer:
This link will download the installer:
Running it will look like this:
Click 'Install'. And wait until:
You can now start Mathcad Prime 12.