2 specific secondary drop down lists can be chosen after chosen specific option in primary

You're not stupid. It's a pretty involved sheet... I'm actually a bit uncertain still how all the pieces fit together.

I think I'm beginning to understand how it fits together... The names you've given your lists in z69LSizeChart could be shortened to match what is in 016. For example STL2101OT_otherstuff could lose the underscore and everything following it. Then your list names will match exactly the data in O16. Next, you can copy the formula from O16 into the data validation for D20 and surround it with an INDIRECT statement.

It should look something like this:
=INDIRECT(CONCATENATE(LEFT(D16,2),(MID(D16,4,7))))

Once your list names are updated to match you should be good to go, and can even eliminate O16.

Can you help me once more? I don't know why cell F20 (the first worksheet of my attached excel) doesn't work. I want the excel output a value referring to the Pantone code for the corresonding primary size on 69LSizeChart worksheet, after I have chosen D16 and D20 on the first worksheet.