The program imports an assembly (either implicitly with the import statement or explicitly with the /reference option) that depends on another assembly. The other assembly cannot be found because it is either not referenced or it does not exist in the specified location.
One possible cause of this error is moving one assembly to a new location without moving the assemblies on which it depends. Another cause is not referencing assemblies on which other assemblies depend.
To correct this error
Make sure that the assemblies required by the program exist.
Check that the location and name is correctly specified for each required assembly.
Make sure to explicitly reference assemblies that are required by other assemblies but are not imported by the program.