Please note this article is specifically referring to the SA100 fetch facility. If you are receiving the error 404 for VAT submissions, please see the link for Common MTD VAT errors
This error is returned when the information for a specific taxpayer and for a specific year is not available. This could, of course, be because the taxpayer is not registered for self-assessment but could commonly also be because HMRC have not yet processed the data into the self-assessment APIs to be fetched.
The process starts on 6th June following the end of the self-assessment year. HMRC process approximately 1 million tax payers' data each night and data for all tax payers should be available by the end of September. Unfortunately, there is currently no means of knowing specifically when a particular tax payer's data will be available.
If you receive this error message when trying to use the fetch APIs you can try again at another time. As above all data should be available by the end of September. For example, data for all tax payers for the 2022-23 tax year (ending in April 2023) should be available by the end of September 2023. HMRC explains the process as follows:
"Data is only available for taxpayers who are registered for Self-Assessment and reported benefits in the given tax year.
Also, data is only available once PAYE reconciliation has completed. This is the process that determines whether the individual has paid the right amount of tax and, for PAYE-only taxpayers, results in a P800 in the event of overpayment or underpayment. Reconciliation starts on or around 6th June and runs for around one million individuals per working night. It has usually completed by the end of September. Unfortunately, it is not possible to know when reconciliation will be complete for a specific individual. If we are expecting a P11D, reconciliation will not start for that individual until we have received and processed the P11D. If we receive a P11D unexpectedly, we will re-run the reconciliation.
Data is no longer available once we have received and processed the corresponding Self-Assessment return."