A tricky one, this is a phrase with lots of meanings. I guess in your context it means you fail to complete a task on time, or make no preparation for an important event. 表演要开天窗了 : The performance's screwed up (because the actors make no preparation or someone forget to bring something important for the performance.) and you can also use this phrase when you are not well prepared for your presentation. You can also see the phrase in the idiom:打开天窗说亮话 it's like "let's get it straight", but the subject is usually unpleasant.

对旧时报纸新闻被当局检查删除而挖去已经排定底版的一部分的戏称。

Very likely it is the case that you had left blank of an essentioal part in the document.

我用汉语回答你，可以吗？你做了a business plan, 老板看了之后不见得都赞成，也不见得都反对，最有可能的是，老板觉得你的计划有一部分是可取的，他不忍心抛弃，还有一部分是他万难接受的，他必须抛弃，他就把他认为该抛弃的那部分给删除了，这就相当于在一面没有窗户的墙上开出一扇窗户，所以叫“开天窗” 还有一种可能，就是你看中了别人的business plan上的某一段，你把那一段扣下来用到你的business plan 上，这就等于你给别人开了天窗。

I think the answer of @Chen Kai is right. Because you mentioned "... in the business plan". If you not mentioned this, I think there are many meanings of 開天窗.