If your bank doesn't allow you to sign in to authenticate yourself, you will need to do so via microdeposits.
After entering your routing and account number, Bonsai will send two transfers of between $0.01 and $0.99. These can take 3-5 days to reach your account.
After these appear in your bank account, visit https://app.hellobonsai.com/settings/payments, and click the green link:
And then enter your micro-deposits: