Locking cells based on cell values

hey i am looking to have the cash on hand reconciliations do the same thing that the bank account reconciliations do. Once the difference is 0, then you can lock the reconciliation by clicking the button.  The reconciliations are the tabs from Oct  to Jun. Also, can we add an if coding so that the bank reconciliation can not be locked until they put a date in? Currently it says Please enter statement date in red but I want my staff to have to put in the date before it will actually lock.  Also, if the date has not been put in can a message box pop up saying that the bank statement date has not been entered?

Thanks!
Bank-rec-template.xlsm