12 week average for a designated column

Hello, I'm trying to write a Beastmode that calculates the 12 week average. The column I want to average is "Net Cost" and the date column is "Week Start Date".

The end goal is to have a beastmode that I can use as a value on a table that shows each distinct "Week Start Date" and the 12-wk-avg of Net Cost preceding that Week Start Date.

Even though it's not working, I'll attach a screenshot of the beastmode as it currently stands.

What am I doing wrong?

Thanks!