Hi. Go into the 'Site Configuration' tab within Site Admin, click the 'New' button above the Parameters list and add the parameter REPORT_MAX_ALLOWED_SIZE.
The default value is set to 1000. While a report it being generated, ALM can only estimate the number of pages. Therefore it cannot be guaranteed that the final generated report will not exceed this number of pages. To leave the maximum number of pages as unlimited, set the parameter to -1. However, this is not recommended as it may negatively impact on your server's performance. To avoid affecting the performance of the server, you may want to increase the number from 1000 until you are able to add the report to the Analysis Tree as intended. I hope this helps!