If your bank link for ACH debit fails, it is usually due to the connection to you bank having been changed. Examples of things that might cause this.
1) Password has been changed on your bank account
2) The bank is asking for a one time password where previously it did not. This is sometimes known as MFA and you will need to enter an extra number that is sent to your email or via text to your phone.
After a failure, you will see a red error message (below). Click on the Relink link to log back into your bank to refresh the connection. After logging into your bank, the connection will be restored and you can use ACH debit as normal.