What sort of syntax errors, and could you give an example of what you have tried. Is it blank or NULL? Sorry, never dealt with that type of database, so I don't what is being returned.
Typically if it is a NULL value you would do something like:
IF ISNULL({SOP_ITEM.DUE_DATE}) THEN
{SALES_ORDER.DELIVERY DATE}
ELSE
{SOP_ITEM.DUE_DATE}
I am not sure how I would handle it being blank as Crystal would probably complain about a datetime value being needed.
You might try setting the null values to a default value of some sort, then you could check for that value in the formula instead of the null. You can set this option at: File/Report Options/Set Null Values to default
Hope some of this helped