Using Dependant Validation. Show error or blank if Inital Validation Changes

Example Attached:

I have set up a data vlidation & then a dependant validation.
What I now want to do is:
When I select a different initial Data Vaidation value
that the cell which picks up the Dependant Validation value - goes blank or throws an error or something
until I select the appropriate dropdown.

This is to ensure that the appropriate Head & Sub are always used.

The solutions I have found online are all based on VBA - which I know nothing about.

Thank in Advance