Hi,
Welcome to our community! 
Please download the .NET Framework 4.7.1 (Developer Pack) from the official Microsoft website and install it. Here is the link:
https://dotnet.microsoft.com/download/dotnet-framework/net471
Maybe you’ll have to reboot your computer. Then launch Unity again and open one of your scripts.
Did this fix the problem for you?
See also:
- Forum User Guides : How to mark a topic as solved