Two measures cannot be cross joined

Hello, I've completed this tutorial: https://community.intersystems.com/post/deepsee-period-date-vs-same-peri...
I've used the "year" level instead of a pivot variable.
I want to calculate something like the following variation: 100 * ([DateOfSale].[PTD] - [DateOfSale].[LYPTD]) / [DateOfSale].[LYPTD]
I create a new calculated member, under the same dimmension with the above expression.
I put the new calculated member under the Count measure and it tells me that "Two measures cannot be crossjoined"
What am I doing wrong?
Can you redirect me to some explanations and maybe a solution?
Thanks!