Question:

How to subscribe to reports in Salesforce Lightning?

Answer:

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.

Related Documentation:

Salesforce: Subscribe Users, Groups, and Roles to Reports in Lightning Experience