I'm missing you with the stars up in the sky.

语法没问题,意思有问题.
with the stars up in the sky 作状语,修饰谓语 am missing,但跟谓语又毫无关系,意思很别扭.如果你想表达：星星布满天,而我在想你,不需要用 with,直接简单地写成：The stars are up in the sky,and I'm missing you.或 The stars up in the sky reminds me of you.或 Seeing the stars up in the sky,I start to miss you.

别人既然用的是一般进行时，你们怎么能用每当……的结构去搪塞别人呢！
i'm missing u,as the stars are rising in the sky

i'm missing you when the star shining in the sky.这个比较通顺

至于语法是没有什么问题的，“每当仰望星空漫烂之时，我就想起你”。关键在于后面的介词“up in”用法有问题，介词+介词是不对的，必须想办法去掉其中一个介词。可修改为：
I'm missing you with the stars shining in the sky.或I'm missing you when i see the stars in the sky...