After effects MP4 render problem
I think you might be confusing pixel aspect ratios with image aspect ratios.

I suspect you want square pixels - ie a pixel aspect ratio (PAR) of 1:1

Most HD videos are 1920x1080, which is an image aspect radio of 1.778 to 1.
Most HD videos have a pixel aspect ratio of 1:1.

Is your desired image size actually 1280x720 ?  I suspect it is, rather than 720x1280 (720 horizontal x 1280 vertical).

Non-square pixels are only usually used for SD widescreen videos - more of a legacy requirement these days.  Unless maybe you're designing for a specific custom display like a big advertising screen or something like that?