If then else statement yielding blanks

Hi all,

I am trying to use an if then else statement but am getting a blank (" ") return on some cells even though it should be 0 when it doesn't match any criteria. I have attached the workbook. The column highlighted in yellow is where I am trying to do the calculations and the column in orange are my desired results. Thanks in advance for any assistance you might be able to offer me!

Best wishes,
uni student