Not much information here to go on, but a couple of things stick out.

One, the function  declaration should probably be:

public function tax(status as string, income as currency) as currency

This would insure the return  value is of the type  currency.

Lastly, there should be a line executed before leaving the function that sets
the return value,

tax = (value)

Be aware that 'status' is a property used several places in VB, so you might
want to name it 'TaxStatus' instead.