First thing : Try running as "Administrator"
1) Please Change your application pool setting.
Keep the steps:
IIS manager open
Click 'Application pools' (Application pools list)
Select your application pool
Right click your application pool and select 'Advanced settings '
Change 'Enable 32-bit application' from false to true. (When you create application pool Enable 32-bit application default assign false)
2) Right click on the project you created the dll and the new referencing project, then select properties. Under Application, check the target framework and verify that both have the same framework, some dll projects tend to select 'client profile' version of the framework by default, which tend to give the error you are having now.