I'm busy during the day so I can't reply. I'll reply at night.
I can't reply during the day as I'm busy. I'll reply at night.
昼間は忙しいと言いたい場合は I'm busy during the day って
表現が良いです。
返信出来ない場合は I can't reply と言います。
忙しいから昼間は返信できませんと言い換えたいなら後者の
I can't reply during the day as I'm busy がおすすめです。
I am busy during the afternoon so I can’t reply. I’ll reply tonight.
I’m busy this afternoon so I won’t be able to reply, but I’ll reply tonight.
昼間 - afternoon
昼間は忙しいはbusy IN the afternoonかbusy DURING the afternoon
~~ので - because ~~, due to ~~
お腹空いてるので - because I am hungry
天気悪いので - due to the bad weather
忙しいので - because I’m busy
返信- reply
Casualの時は”get back to you”
あとで返信する - I’ll get back to you later, I’ll reply to you later
返信できません - I cant reply, I can’t respond
日本 
イギリス 
