Cannot check the "Retainage" box when creating a complex PO document style

I'm trying to introduce retainage functionality in Purchasing, r12.2.5 but I can't check the "retainage" checkbox. 

I have entered the retainage account combination in Purchasing Financials Options.

I have set the system profile value PO: Enable Services Procurement to "Yes" at the site level.

I have updated a supplier's site -  invoice management - with 20% retainage.

I created a document style of "Complex PO", but I cannot check the retainage box.  It is grayed out.

I notice that setting the profile option is what makes the "Complex Payment" section appear, but it is still grayed out and I don't know why.