Formatting spreadsheet to automatically update time per column entry

Hello, all. I have a fairly simple spreadsheet layout, consisting of 3 lists. I'm concerned with the 3rd, where I need Column A, Date, to automatically add the current date and time when the corresponding row in Column B, Activity, is modified.

I've followed a few tutorials, and while this works perfectly fine in a new document, I've found I:

A, don't know how to invert the column relationship

and

B, cannot get even the basic, otherwise functional code to work in this spreadsheet.

I'm losing my mind. I've looked everywhere for what could be the issue, but haven't found anything. The file is saved as .xlsm, I am entering the VBA code into the little pop-up window into the List 3 section, not ThisWorkbook or a different one.

I apologize in advance, as I'm quite new to Excel and have only had very basic experience prior.