Background color not showing

The background color of a comp WILL NOT RENDER. The only use for adding a background color is to provide edge color adjustments to alpha channels when they are rendered pre-multiplied with color. There is no other practical use for a composition background color.

If you want a colored background add a colored solid to the composition and put it on the bottom layer.

I have never met anybody that has successfully just opened up After Effects and started working with any kind of success. If you are new please start here: Basic AE

You also need to study up on video standards and video compression. When you first start out none of you compositions should say Custom in the Composition settings and you should not modify any of the output module or Adobe Media Encoder rendering presets.

If you get a tranpsarent background like a grid  (  checkerboard )  there is a button called Toggle transparency grid ( it looks like a grid )

let us know if that helps.