The amount of reports you can create per day is limited for your organization. Contact your Customer Success Manager in order to ask for a raise.
With this API call, you can request the creation of a custom report. The result will not be returned in the call itself, but available a few minutes after. The reason for this, is the large amount of data that possibly needs to be queried, or that not all data is already available at the time. For example: the report will be generated at least 20 minutes after the end time that has been set to be sure all data coming from our CDN providers is processed.
You can always check the status of the report generation via the GET /reports/ API call. This will also return a link where you can find the report once it has been generated.