A BOM Journal is an inventory journal, and all lines can only be posted in the Inventory unit of measure.
To make one PP-MF item, AX needs to post consumption of 20cm (or less) on your PP-PN-PN lines.
20cm = 0.05 Pcs. I expect the decimal precision on your 'Pcs' unit is set to zero; therefore AX rounds that to zero decimal places.
0.05 rounded = 0. Therefore, consumption of these PP-PN-PN lines is zero Pcs.
What happens if you create a BOM Report as Finished for one thousand of your PP-MF item?