To verify the number of allocated and used licenses:
- Navigate to Setup->Build->Installed Packages
- Verify the license status, expiration date, allowed and used licenses
If the Allowed Licenses column says Unlimited then you are either in a sandbox org, or you have a site license.
If it shows a number, you can assign licenses to individual users by:
- Clicking the Manage Licenses link next to the Decisions on Demand app
- Using the Add Users and Remove Multiple Users buttons to modify the licensed users.
You can also click the Remove link next to individual users.
If you are assigning licenses to many users, there are two options to simplify the task:
- Create a User list view and select Add All Users
This will add all users selected in the view -- not only those visible on the screenNote that the Create New View option currently does not appear to work in Lightning
- Do a bulk upload using Data Loader or a similar tool
See here for details: https://help.salesforce.com/articleView?id=000267828&language=en_US&type=1