A synthetic backup can only be created in a policy.
You can use the following methods to create a policy that contains the necessary job templates for a synthetic backup:
Make a copy of the synthetic backup policy example and then modify the job templates to suit your specific needs.
See Creating a synthetic backup by copying the example policy.
Manually create a policy, and then add the necessary job templates for the synthetic backup feature.
See Creating a synthetic backup by adding templates to a policy.
Before creating a synthetic backup, review the requirements for synthetic backups.