To avoid overloading the system, there are built in limits to the number of reports that can be run on a given site each day, week, and month. If you try to schedule a report that exceeds the limit, you will get that error.

The specific limits are:
Daily: 5
Weekly: 20
Monthly: 25
Other: 25

If you run into those limits, you may need to review what reports you are running and when to better manage your reports within these limits. Only 5 reports can be scheduled to run on a single day. For example, if you have 3 daily reports and 2 weekly reports that run on Monday, then the 5 reporting slots for Monday are maxed. You'll need to schedule the 2 weekly reports to run on Tuesday instead.