Like なくてはいけない, なければいけない is a structure that implies that something ‘must be done’. However, it is important to remember that this is actually a double negative, and therefore means ‘If (A) is not done, it cannot go’, or more simply ‘(A) cannot not be done’.
To make this construction, the auxiliary verb ない will need to be conjugated with ば, creating なければ (if not). After this, いけない (to not be able to go) will be added to the end.

• 来月（らいげつ）に友達（ともだち）の結婚式（けっこんしき）に行（い）かなければいけない。
  Next month, I have to go to my friend's wedding. (Not going to the wedding is not going to work)
• 車（くるま）は駐車場（ちゅうしゃじょう）に止（と）めなければいけない。
  Cars have to be parked in parking lots. (Not being parked in a parking lot is unacceptable)

As learned in the なきゃ lesson, なければ may be shortened to なきゃ. This structure is frequently used either with or without the added いけない.

• ここで話（はな）している場合（ばあい）じゃない。息子（むすこ）を迎（むか）えに行（い）かなきゃ。
  This is not the time to talk here, I have to go pick up my son.
• 宿題（しゅくだい）で、調（しら）べなきゃいけない事（こと）があるから、パソコン貸（か）して。
  Since I need to look up something for my homework, would you please lend me your computer?