When linking a bank account using your online banking credentials you may be asked to enter an authentication code or a code received via text message yet there is no place to enter the code when linking the bank.
If this is happening you might have two-factor authentication enabled on your bank account. This is a common way to keep your bank account safe. If you are linking your bank account to your Stockpile account using your online banking credentials you won't see a place to enter that code.
You would need to attempt to link to a different bank, and you will want to make sure this bank is a basic checking account. If you don't have another bank that you would be able to link, then you can still fund your account using a debit card.