If your bank account number is formatted like this: XX-XXXX-XXXXXXX-XXX
The first two groups is bank and branch number.
Account number will be the account number and suffix, the second two groups.
Some banks give you 3 digit suffix but use only the last two digits.
Basically, if your account number is 12-1234-0123456-000, you'd enter it as bank number or code "121234", account number "0123456-00".
If trying to pay with balance, perhaps you need to add a card as back up and also select your PayPal balance as the preferred payment method for online purchases this way: Go to Account Settings (gear icon) > Payments > Online Purchases