When you save an audience rule, you have the option to enable Audience Trend Analytics which will automatically take a snapshot of the audience size every day at midnight. Learn more about Audience Trend Reports.
You are able to save a maximum of 50 audience rules with audience trend analytics enabled.
If you are unable to save an audience rule, you may have surpassed the 50 audience rule limit.
Disabling the audience trend analytics checkbox should allow you to save the audience rule.