Changing column name and finding mean and standard deviation in Spyder (Python 3.9.7)

Question

I'm working on a project for class, and y'all have been so incredibly helpful!

Anyways, I used this code:

SO21= BI21[BI21["OnHand"].eq(0)].groupby("BrandID")["InventoryDate"].nunique()

And it generated this series (please bare with me I'm still learning how to best input my information here for you to see):

enter image description here

I then wanted to find the sum, mean, standard deviation, and standard error and used this code that we learned in class:

SOSummary19 = SO19["InventoryDate"].agg(["sum","mean","std","sem"])

It returns a KeyError: "InventoryDate'

My goal is to take those measurements based on the numbers in the 'InventoryDate' Column from the image I posted, and I would also like to rename 'InventoryDate' to 'DaysStockedOut'

Thank you in advance for your help, you are ALL amazing!