Simulation in value driver tree : Exit function / fox formula not available

Hello,

I am trying to create a value driver tree with simulation.

I create different planning functions (with Fox Formula and Exit function).

These functions work in BPS0. But I am not able to select these functions in the design of the value driver tree.

Example of a FOX function:

*Operand Display {Key Figure Name,Currency,Key Figure}

DATA QTY TYPE F.

QTY = {0QUANTITY,#, VDT_PRODVOLUME}.

{0AMOUNT,EUR,VDT_PRODCOSTS} =

{0AMOUNT,EUR,VDT_PRODCOST} * QTY.

This function can not be selected in the value driver tree design for the measure VDT_PRODCOSTS.

I don’t understand why? Somebody could help me about the reasons of this phenomena?

Thank you in advance

Best regards

Jean.