.NET DLL problem(s)

Hi, everyone.

I'm and another teammate experience some issue while 2 other teammate doesn't,
We are all using unity 5.4.0f3.

The problem is that some of the .net classes or method in .net classes are missing
but only when I'm tring to build the project, when I'm debugging in unity it's working and when I'm looking in visual studio everything fine and the references work.

exp :

1) error CS0117: 'Assembly' does not contain a definition for 'GetAssembly'

2) error CS1061: 'Type' does not contain a definition for 'IsAbstract' and no extension method 'IsAbstract' accepting a first argument of type 'Type' could be found (are you missing a using directive or an assembly reference?)

3) error CS0246: The type or namespace name 'TcpClient' could not be found (are you missing a using directive or an assembly reference?)

code example :

IEnumerable types = Assembly.GetAssembly(typeof(Model)).GetTypes()
.Where(type => type.IsClass && !type.IsAbstract && type.IsSubclassOf(typeof(Model)));

or

IEnumerable types = System.AppDomain.CurrentDomain.GetAssemblies()
.SelectMany(s => s.GetTypes())
.Where(p => typeof(Model).IsAssignableFrom(p));

I tried to remove the unity from the PC and install it again but it didn't work,
and change from Assembly to use AppDomain but no luck in there because :
"The type or namespace name 'AppDomain' does not exist in the namespace 'System' (are you missing an assembly reference?)"
and I tried to change the Api compatibility Level to .NET 2.0 but still didn't do any good.

What am I missing ? do I need to install something in particular to make it work ? where is the problem ?