If your RDS cluster was created without performance insights enabled you will need to enable it using AWS CLI.
aws rds modify-db-instance \ --db-instance-identifier <YOUR DB INSTANCE ID> \ --enable-performance-insights
You can add the parameter
performance-insights-retention-period to the command
modify-db-instance if you want to change the default amount of days (7) that Performance Insights data will be retained. That is the maximum amount of time allowed for using performance insights with no charge. You can find more information about pricing in the Performance Insights Pricing page.