If you forget the cash app PIN and it is not working then you will have to reset Cash App PIN. 
Following are the steps that you need to take to change the Cash app PIN
 
 Open the Cash App and open the profile icon.
 In the Profile, menu select the the “Privacy”
 Then click on the “Change Cash PIN”.
 choose the  ‘Forgot Cash PIN.’ option reset the Cash app PIN
 After this, you will receive a verification code
 Enter the code, to Set-up a new Cash App PIN
 iPhone users are allowed to reset the PIN using a touch ID, as well.
 You are required to enter the new PIN twice to complete this task.