Does it mean 'posts' as in jobs or things 'posted' on social media? I guess it means 'posts' as in jobs or roles. Then I would probably use 'where' so 'Posts where I collaborated with her'. But then what does 'in it' refer to? Even if you remove 'in it' and change 'that' to 'where' this sentence is still incomplete. 'Posts where I collaborated with her' is a noun clause, the rest of the sentence would need to say something more about them. I think I would need to know more to make any suggestions here!