It is possible that the issued consent does not permit this operation. An AIS-Consent defines three levels of access. The first level allows access to “accounts”, the second to “balances” and the third to “transactions”. Combinations are possible and a selection of either “balances” or “transactions” grants access to “accounts” as well. A consent granting access to e.g. accounts and transactions does not allow to call the balance endpoint.
In this case you should receive an error.
Solution: Ask the PSU to create a new consent allowing you access the appropriate endpoints.
Another reason might be that the consent status is not valid. A status will e.g. expire automatically when the “expirationDate” defined within the consent is exceeded. It might also be possible that an error occurred while performing SCA. For more detailed information see FAQs on Strong Customer Authentication.