yes i checked the resource member id, that was fine but the issue was with the financial plan type.. i was on trying to use the budget type code in R12, which now, i think, is not supported. Please correct me if I am wrong in understanding this. When i provided the fin plan type id, it worked.
Looks like your budget type might have been upgraded to financial plans. Oracle Projects has two models when it comes to budgets. One is the traditional form based budgets, identified by budget types. The other is the Financial Plans, which is used in Project Management Module. The older model ones are referred to as budget and the newer model ones are referred to as Financial Plans which provides several more additional functionality as compared with the traditional budget types.
Both are supported by oracle, however, once a budget type / budget version is upgraded to financial plan types/versions you will not be able to maintain them using the forms interface, you will have to use the self service pages only for those.
It appears that your budget type / budget version might have been upgraded to financial plan types, which is why you had to pass the fin_plan_type_id (Financial Plan type id) for the API to work.