Hi Murzuf,
Welcome back to our community! 
Unity works with 4.7.1 only. Not with 7. 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.
If you use a Mac or Linux, download the latest MONO Framework.
Did this help you fix the problem?
See also:
- Forum User Guides : How to mark a topic as solved