Why can't I place a banner on a shield in Minecraft Bedrock Edition?

That doesn't exist. Sorry. You cannot do that yet in Minecraft. Maybe one day you will be able to, but for now you can't.

Actually. Not to burst anyones bubble, but this is very real and you can customize your shields but only in Java. So alternatively, only for PC. And if you're playing Java the banner should be in the first square it the middle row and the shield should be in the second square of the middle row.

This cannot be done in Bedrock edition, unfourtunatley. Perhaps a behavior pack could be made to enable it, but it is not vanilla.

I’ve tried to do this on Bedrock edition, Java, and pocket edition and you can’t do this. Unless it’s not on PS4 or Xbox 1 then as far as I know you can’t do this yet.