F-65 Special G/L transactions not defined for bills/exch.and down pmnts

I used F-65 try to park a document with postkey 09/19/29/39 and system show

"Special G/L transactions not defined for bills/exch.and down pmnts"

I know the reason is the SGL code.

But why ?? why do the control like that?

Checked the code below :