I found this in a phrase: 隠れていなくて. I can translate it simply as "aren't hiding" or "were hiding"? By the way, is it correct or is 隠れていません better?
By the way, the whole phrase is
It's fine. 〜なくては and its contracted version 〜なくちゃ are commonly used to mean roughly "must/should do", with following verbs such as いけない or ならない omitted. In your example, the omitted verb would go before the particle な:
I think the speaker is telling the listener that they'd better hide behind someone strong, like the speaker. It sounds like the speaker is telling the listener that they're weak.
Compare 〜なければ and its contracted forms 〜なけりゃ／〜なきゃ, which can similarly be used with the following ならない (or いけない, etc.) omitted. The following verb is literally the consequence of not taking some action--in this case, hiding--and it's almost always something negative. It can be omitted because the listener can imagine what it is.