I think "StartDate" means "FiscalYearStartDate", in which case the
formula barbetta suggested would calculate the week of the fiscal year
based on the fiscal year start date. It finds the number of days
between the fiscal year start date *you provide* and the date in column
B of worksheetA, then divides by 7 and rounds up to the nearest integer
to give you the correct week based on your fiscal year. Maybe I'm not
understanding this correctly, but using that formula is much simpler
than the VLOOKUP you're trying to do.

It appears the fiscal year start date is currently at WorksheetB!$A$2,
so

=IF($B2="","",ROUNDUP(($B2-WorksheetB!$A$2)/7,0))

should do it. Let me know if I misunderstand.