Why is 'my' not translated here? Because when I add 내 to the sentence it's not correct.

Is 친구한테 not appropriate in this case if using 반말?

Why cant I use 제가 in here instead of 저는?

I put "나는" instead of "저는" and was marked incorrect - why is this?

The "my" is missing in the accepted answer. If you hover over "to my friend" it shows some form of my in all three options but if you put any of them the answer is not accepted.

Is the order, "I to my friend a letter send"?