Undeterminable as you haven't explained what you're trying to achieve.
But you either want this

=IF(AC4 > 45,"",IF(AC4 > 29,"Must",IF(AC4 > 19,"Should",IF(AC4 > 9,"Could",IF(AC4 > =0,"Will Not",IF(AC4="Legal or Fiscal","Must","OTHER"))))))

or this

=IF(AC4 > 45,"",IF(AC4 > 29,"Must",IF(AC4 > 19,"Should",IF(AC4 > 9,"Could",IF(AC4 > =0,"Will Not",IF(OR(AC4="Legal",AC4="Fiscal"),"Must","OTHER"))))))