Two things to make it work.
1. Make your camera Orthographic
This will help you avoid distortion in corners, and will make it 'infinitely flat'.
It's like using an infinitely long lens.
2. Make the array 1:1 aspect ratio
Your chainmail array is ever so slightly taller-than-wide.
Move the rings a bit along the X axis, so they fit nicely.
To bake into an image you need to have it selected in the Node editor
you don't have to connect it to anything
just an image texture node then select the image you want to bake on and make sure that the node is selected