To assign the app licenses to users, you need to follow the below steps -
- From Setup, enter Installed Packages in the Quick Find box, then select Installed Packages to find the installed package that has available licenses.

- Click the Manage Licenses link next to the Mass File Migrate name.

- Click "Add Users".

- To select individual users, use the checkboxes. Selected users are listed in the Selected list. When the list includes all the users to whom you want to assign licenses, click Add.
- To select all users for the current view, click Add All Users button, then click OK.

PS: View Mass File Migrate Salesforce file migration app on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=e9874a66-2001-4e3d-8a29-3a2ed7f584c4