Not able to define transfer rule

Hi, I am working on SAP Netweaver 7.0 and is trying to define transfer rule for Keys. I want to calculate revenue as rev = Price * Quantity

I am getting error message as "Formula ELement does not match type". The Keys are defined as below:

REV5 datatype as CURR type Amount

Quantity datatype as QUAN type Quantity

PRICE datatype as CURR type Amount

I am using Transaction RSA1OLD to work on old SAP BW.

Below is the text in help with which I m confused.

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Formula element does not match type /BIC/OIIO_REV5

Message no. SFB525

Diagnosis

The selected formula element is of the type BOOLEAN. At this point in the formula, however, the system expects an element of the type /BIC/OIIO_REV5.

Procedure

Enter a formula element that matches the type /BIC/OIIO_REV5.

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Seeking help one more time..

Thanks in advance,