You can't eat while laying down on the floor. If you're going to eat something, you need to sit up.
"You can't eat while laying down on the floor. If you're going to eat something, you need to sit up."
「床に横になっている時は食べちゃだめ。もし何か食べるなら、ちゃんと座って。」
* eat: 食べる
* while: 〜〜しながら、〜〜している間
* lay down on the floor: 床に寝そべる
* if: もし
* sit up: (背筋を伸ばして)座る
ご参考になれば幸いです。