We have come across this once or twice before. Although the framework is installed on your PC it is not an active feature.
Below please find advice from Microsoft on correcting / overcoming the issue:
The .NET Framework is an integral part of many applications running on Windows and provides common functionality for those applications to run. Normally, before running/installing such applications, we need to enable .NET Framework from the Control Panel on the computer. So, you may first check if the .NET Framework 3.5 is available in the Control Panel on Windows 10 Technical Preview and if available, you may enable it from the Control Panel to install it on the computer. To check if the.NET Framework 3.5 is available in Control Panel and to enable it, please follow these steps:
a) Press “Windows Logo” + “R” keys on the keyboard.
b) Type “appwiz.cpl” in the “Run” command box and press “ENTER”.
c) In the “Programs and Features” window, click on the link “Turn Windows features on or off”.
d) Check if the “.NET Framework 3.5 (includes .NET 2.0 and 3.0)” option is available in it.
e) If yes, then enable it and then click on “OK”.
f) Follow the on-screen instructions to complete the installation and restart the computer, if prompted.