'Subscribe' Button Does Not Appear on Salesforce Reports — Enable Report Notifications

In Salesforce, users can subscribe to reports to receive scheduled email or mobile notifications. If the Subscribe button does not appear on a report, it is because the Report Notifications feature and Salesforce App Notifications settings have not been enabled for the organization.
This article explains how to enable these settings so that the Subscribe button becomes visible on reports.
Note: Report Notifications are enabled by default in Lightning Experience and cannot be disabled once active. This article's setup steps primarily apply to Salesforce Classic. Notifications are limited to 5 subscribed reports per user and this limit cannot be increased.