The 2015 program will not be retired/sunsetted/discontinued until 5/31/18. If you going to purchase additional licenses you may want to upgrade first.
Resolution for 'additional licenses issue: 'Increase Number of Users' available: Yes.
The additional licenses issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the additional licenses issue yourself by looking for a resolution described in the article. If it is a complex additional licenses issue or you are unable to solve the issue, you may contact us by clicking here or by using other additional licenses support options.
For more information about resolving issues related to additional licenses in Quickbooks/QBO or to get support for the issue 'Increase Number of Users', click the link below: