When an admin receives a request to add a new user, they may encounter an issue if there are no licenses available to assign to the user.

Why am I (an admin) unable to assign a new user a Salesforce or Salesforce Platform license?

The first thing an admin needs to verify is whether they have any licenses available to assign.  You can find this by going to Salesforce Setup >> Quick Find: Company Information.  It will look like this:

An admin user will need a Salesforce license, and a non-admin user will need a Salesforce Platform license.  The column the admin needs to concentrate on is the Remaining Licenses.  This will tell you whether you have used all of the license or not.

Once the admin has verified that the issue is not due to a lack of licenses, proceed to the Quick Find and enter Installed Packages.  Locate the package named Matter Management and click on the Manage Licenses link to the left.  

The admin will need to focus on the Allowed Licenses and the Used Licenses section at the top right.  If the Used Licenses number is less than the Allowed Licenses number, then a new user can be assigned to the package.

However, if the numbers match, then the admin must verify if all of the licensed users currently assigned to the package are active (the column outlined in green).  If there is an inactive user, the admin can remove the user from the package by clicking on the Remove link on the left side (the column outlined in red) in that same row.