The reconciliation feature is only possible if a bank was added via the Instant Verification method that enables the connection with your bank and makes it possible to import the bank statement feed.
The system will prompt you to re-connect the manually added bank account once you click "Start" for the respective card on the Reconciliation page. Click "Re-connect bank account" on the next page:
Then, click on the "Agree and continue" button and select your bank to re-add it.
The following link may be helpful for you: How do I add a bank account?
Also, if the bank was added but the permission to import the statement records was not granted, you would need to re-add the bank account as well.