Other comments seem to be confusing ensure for baby formula. The remainder of those comments are as misinformed, disregard them.
Ensure is a nutrition supplement or replacement for adults. It is a pharmaceutical product intended for use by people who have difficulty taking in enough food. Examples would be cancer patients and those with feeding tubes.
Breast milk is a substance produced by humans for infants and young children. It is composed to provide all needs for a baby and then to supplement during weaning. Notably, it will be much higher in fat. It also contains only those proteins which are digestible to an immature human gut.
Do you have an underlying question?
If you are considering replacing one with the other, no, it is not advisable. It will lead to complications possibly death, sooner or later.