You receive this error when you have added reference to the said dll in your project from Microsoft Enterprise library 4.1 (or any version for that matter) and when you try to build your project. The full error detail will read something like this along with your other project details:
Assembly generation failed -- Referenced assembly 'Microsoft.Practices.EnterpriseLibrary.ExceptionHandling' does not have a strong name
If you have added a .snk key in your project to sign your assembly with, remove it, as this error may occur if you are using unsigned dlls of Microsoft Enterprise Library while your own dll will be signed. So removing the key will remove this error, on the other way round, you must use the signed dlls of enterprise library.