In general the license capabilities are default assigned to admin roles.
If you want to add the capability to users to handle splunk license, you have to add the following capabilities to the roles you have created to:
license_edit : Lets the user edit the license.
license_tab : Lets the user access and change the license. This attribute is deprecated(Although it is saying that is deprecated, I've seen this capability at Splunk 8.0 as well)
license_view_warnings : Lets the user see a warning message when they are exceeding data limits or reaching the expiration date of their license. These warnings appear on the system banner.
go to menu Settings - User and Authentication - Roles
Select the role/edit and then select the license capabilities hitting the check the box and save to complete the configuration: