Is it possible to...make a cell a required field to fill in if there is an
entry in another field. As a type of data validation.

i.e In B3
If B6 is not blank, make B3 a mandatory cell to fill in.

B6 thru to B22 have data validation to choose from a list True, False, NA

looking for, if any of the cells in that range are not blank, ensure they
enter the file number in cell B3

Thanks