If you get this message after installing Mathcad Prime 12:
Then you need to install .NET 8.
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_net8.html
Clicking on the Download link will take you to the Microsoft download page for .NET 8. 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.
You can now start Mathcad Prime 12.