ERROR Error: Uncaught (in promise): NullInjectorError: R3InjectorError(e)[df -> df -> df]: NullInjectorError: No provider for df! code example

Example: ERROR Error: Uncaught (in promise): NullInjectorError: StaticInjectorError(r)[n -> n]: StaticInjectorError(Platform: core)[n -> n]: NullInjectorError: No provider for n!

ERROR Error: Uncaught (in promise): NullInjectorError: StaticInjectorError(r)[n -> n]: 
  StaticInjectorError(Platform: core)[n -> n]: 
    NullInjectorError: No provider for n!