Lack of namespaces when downgrading to asp.net 4.0

Question

I just had to downgrade my ASP.Net 4.5 application to an ASP.Net 4.0 application because our servers do not support 4.5. The way I downgraded was pretty simple:

Project → Properties → Application → Target Structure

When he finished doing things, I restarted the solution, tried to start debugging, but he stopped right after the debugging started, giving me the first errors (which say that they cannot find some namespaces) for a split second and after the errors disappeared, this gives me 127 warnings that everything looks like this:

Now I know what the problem is, but I do not know how to solve this problem. Does anyone know how to fix these warnings?

Thanks in advance

INFO

I am using the Visual Studio 2015 community

Answer 1

Dll , Dll. 4.5, , , .

Answer 2

, ASP.NET. web.config . , ASP.NET .NET 4 web.config .