Hi,
Just remove it from the dll from your project and then try to build the project. Since you don't need that one.
In case you want to include that dll, you should check that, the application pool you are using is compatible with the assembly. I have encountered these kind of errors in my past. Please check that dll is 32 or 64 bit. If it is 32 bit, then open the IIS and select the respective app pool and go to "Advanced setting" and "Enable 32-bit Applications" to "True".
Hope this helps!
S Francis
My ASPDotNet Blog