When editing a Software Product (from Manage > Software or Manage > Software Catalog), after picking a product from the Installed Products list and then clicking on its Manage Licenses button, the Add License Purchase button is disabled and cannot be used.
Ensure that the "Licensable product" checkbox is checked (enabled) to allow licenses to be added. If this is already added but the Add License Purchase button is still not enabled for use, it is possible that this is occurring due to data issues. If this works on other Software Products, then something with the record for the failing on is the issue. Try manually adding the license in the Asset Management user interface by going to Home > Service and Asset Management > Software Licensing.
If Asset Management Solution is not installed, this section will also be disabled and unusable, with the warning "Software License Management [i.e., Asset Management] not installed" will appear. Install Asset Management to utilize this function.
Beyond this, it's not known what may actually cause this. Some possibilites are:
This may have been the result of a failed Data Connector import. If any Software Products or Software Licenses were added by an import rule, double check that these were brought in correctly. If not, they may need to be manually deleted then re-imported to then work successfully.
If Asset Management is installed on a child server in a hierarchy, this can be the cause. Asset is not supported on a child server or when used with stand-alone replication rules. Uninstall Asset from the child server and use this interface only on the parent. Information about this can be found here: