The YouTube Reporting API can take 24 to 48 hours to process reports. For example, if the report is scheduled on May 1, 2021, for that day's specific data, it will be ready on May 3, 2021.
This doesn't impact all parts of the API — some data might still be available same-day. But if this data is missing, the reporting limitation is likely the cause.
Learn more about this in YouTube's documentation.