unfortunately, I don’t see what exactly you tried to read with the FUB you mentioned, but generally speaking, B&R Online Help should be the solution to reading an analog value from a drive.
something that can be easily missed when dealing with DIO and AIO ParIDs is, that the slot number of the card hast to be added to the ParID number.
ACOPOS P3 hast only slot 1, so in this case it is always +1.
e.g. Reading analog input number 2 (on card in slot 1) from ParID: