Visual area map - coloring

Hello,

i have generated a area map visual with some input (earnings, zip code) - this works fine. But i can't set a gradient using a formula - its only possible to give each item a color 😞   

I have navigated to the "format visual" tab -> there is a section "fill colors" - an in this section i can give each item a color, but the formula symbol (to set a gradient) is not available!?

Is this a version problem? ( Version: 2.116.966.0 64-bit (A 2023) - Power BI Desktop)

thanks & regards christoph