How to subscribe to reports in Salesforce Lightning?
Subscribing to Reports in Lightning Experience is controlled by a different set of system permissions than in Salesforce Classic.
Please refer to the following Salesforce documentation to review the required permissions:
Salesforce: Subscribe Users, Groups, and Roles to Reports in Lightning Experience
If these permissions are not available, the "Do Not Auto-Enable New Features" setting may be active in the Org. Please contact Product Support requesting a Salesforce support case be opened to have this setting changed.