Exception:

All subscriptions used – To subscribe to this report, first unsubscribe from another one

Maximum number of Reports or Dashboards, an User can subscribe to is 7in Salesforce. So, the “All Subscriptions Used” “To subscribe to this report, first unsubscribe from another one.” is thrown when an user tries to subscribe to more than 7 Reports and Dashboards.

Salesforce Idea to increase the limit is https://trailblazer.salesforce.com/ideaView?id=08730000000DkodAAC