I have looked through other posts but none seem to answer what I need.
This may not be your exact issue, but I have had this happen for a couple reasons:
Incorrect Framework on Server: Make sure the target framework of the project is supported by the server. Just changing the targetFramework attribute of the compilation element may not be enough as the DLLs for your project may be compiled against the 4.5 framework.
Incorrect Framework of Application Pool: This can also happen if your application pool is not using the right .Net CLR version. Make sure it is using the 4.0 instead of 2.0