How to fix my table code? It gives me an attempt to index field 'Y' a nil value

The code errors because Y doesn't exist, you need to create it before assigning something to it, it's like trying to change the property of something that doesn't exist, first the instance needs to be created in order to be changed.

local D = {};
local E = {};
E.Y = {} -- Create Y
E["Y"].Test = "Hmm"
table.insert(D,E);
print(D[1]["Y"].Test);