Error Select list item: Attempt to get item number

[TIMAI2]

From here:

Form with data like csv in Tinydb

Hi, I search to create an form where the entry data display like a csv(column, line, cellul) to the screen and in Tinydb. below I join a model : [screen]

What you haven't done is made a sublist from your main list of all the 4th items, you then perform the sum on that sublist.