This could happen because of the following reasons:
1. The bank you have an account with is not part of the Account Aggregator system.
2. Your bank account is linked to a different phone number than your savings account.
3. Banks that are a part of the Account Aggregator network currently do not support Joint Accounts.
4. The other bank's servers are down.
If reasons 1 & 2 do not apply to you, could you try again in sometime?