One Way -

Assuming your list of invoice numbers is in cells A1:A1000 on Sheet
and that they are sorted in numerical order -

On Sheet2 enter the following formula in A2
=IF(SHEET1!A2=SHEET1!A1+1,"",SHEET1!A1+1).

Copy this formula down to cells A3:A1000 on SHEET2 and you will have
list of all missing invoice numbers with the other cells all blank. I
you want to remove all of the blank cells, just sort A2:A1000 i
ascending order and your list will be in consecutive cells at the to
of the column

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