The non-admin user does not see the "Subscribe" button in the knowledge portal(kb) and not able to subscribe to the knowledge articles.
The system properties "glide.knowman.enable_km_subscription.roles" manages the users who can see the subscribe button in knowledge portal.
In OOB, the property 'glide.knowman.enable_km_subscription.roles' is set to the role 'knowledge'. Edit this property to show the subscribe button to other role users as well.
Refer the below docs for more details: