The type or namespace name 'Linq' does not exist in the namespace 'System'

Question

I have a wcf service hosted in a website in IIS and I seem to have this issue. In my web.config I have this:

Answer 1

Sometimes it "disappears" just for the fun of it. If you don't need it just delete it.

Else right click your website in the solution explorer and add a reference to it.