find the message number (its V50R 012), goto SE91 and use the where-used button for this message. You will find the program LV50R_VIEWF34
and there the coding part:
IF <cs_postab>-vbtyp NA vbtyp_verk.
MESSAGE i012(v50r).
CONTINUE.
SAP is just checking if the predecessor document type is a SD document, if that is not the case it is not possible to process in foreground. A technical design gap for a long long time already.