The required PIN to add a bank or authorize a bank transfer is your Cowrywise PIN, not your ATM or Bank PIN. You can reset it from your profile if you don't remember it.
Please follow these steps to reset your PIN:
Go to the edit profile page on your app
Swipe to the security page
Click on change PIN
Tap on reset PIN
You will get a mail from Cowrywise with an OTP to reset your PIN
Use the OTP to set up a PIN.