In the Policy setup:-
When you create the Backup templates - there will be an Option in Settings general - Collect Additional Information for Synthetic backup.
Make sure this option is checked for both the Baseline and Incremental templates.
Then you can create the Synthetic Template itself.
Please note - you will only see these option if the Advanced Disk Based backup option is installed and licensed (whether it be through eval or with a valid serial).
There are also Example Policies - if the above option is installed, then choose, Tools, Re-create Example Policies.