It is very common error with .net assemblies. When someone included dlls from outside, in the project, this kind of exceptions are generic.
Here is the resolution.
- Open the file location (MySql.Data.dll in this case)
- Right click on dll and check its properties (it should not be encrypted and should have right access permissions)
- Run the project after assigning appropriate permissions. It should work now.
Even after this if it does not work. Check in web.config that assembly information is added under <compilation> tag.
It is advisable to included new dlls using nuget package reference. It will update its reference automatically.