Is the usage of “know” wrong in the sentence?
No, the use of know is correct in this sentence.
What might be making it confusing for you is the rest of the phrase, in the know. This is actually a phrase that means that someone knows about something, especially if that thing is supposed to be secret.
For example, a company might know that they broke the rules. They wouldn’t want all their employees to know about it, but their top officials probably would all know about it, even if they did not have all the details. In this case, you could say:
- The top officials of the company were all in the know about this cover up.