error solution?

Question

These error are occur in My VIsual Studio 2015. Please sort out it.

 

1) Always give error in my website in linq library Error shows that Linq Library missing or does not exist in the namespace (Linq Library mentioned in namespace).

 

2) Controls not working.

 

3) Red Line Show in C# Event for Each Control (Control declared & Event Generated).

 

4) Server Tag show blue line runtime error. Error Shows that Unrecognized Configuration section authentication.