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.
The default number of audiences that can be saved is 100 audience rules with audience trend analytics enabled. If you reach the maximum and need to save more audience rules, contact your CSM to configure your account to extend the default limit for your account.
Disabling the audience trend analytics checkbox will also allow you to save the audience rule.