Hi and welcome to the Board
The reason your function doesn't work past the first part is becasue the conditions have been met.
=IF(C13<=B3*D3,"IN STOCK","BACKORDER"),IF(B3*D3/C13>=1,"IN STOCK ",ROUNDUP(B3*D3/C13,0))
IF(C13<=B3*D3, then if true "IN STOCK", if False "BACK ORDER".....therefore no reason to do the rest !!
As this is an assignment, we won't provide the answer, but why a ROUNDUP function if you are having an IN STOCK or BACKORDER result, neither of which are a numeric result
What do you want to see in the cell, how many to order, or the fact that they are on back order !