You can't use any function to modify a range in SUMIFS.....and also I would say that WEEKNUM isn't best for this as you may get confusion between years; and also weeks at the start/end of the year may not have 7 days (because week 1 always starts on 1st Jan whatever the day of the week - according to how WEEKNUM works anyway). You can use WEEKDAY
function more easily, e.g. in F5
=IF(OR(A6="",A5-WEEKDAY(A5,3)<>A6-WEEKDAY(A6,3)),SUMIFS(E$2:E5,A$2:A5,">="&A5-WEEKDAY(A5,3)),"")
That uses WEEKDAY
to find the previous Monday and sums anything that is in the last week based on that - so this will work even through Dec/Jan