Hi Ayaskant,
It can be a special case like some one unknowingly added the material in IM and later the IM deleted or some other reason.
If you get this kind of error No active integration model exists in R/3? then you can activate the IM and again deactivate the IM for the Product/Location.
ATP check will be performed in ECC itself.
It is not necessary that only X0 materials are CIF'd to APO. We can transfer most of the material (MRP types). Only differentiate in Planning.
Thanks,
BK.