". . . license not imported because license is invalid"
When attempting to import a Nintex Forms or Workflow license via Central Administration > Nintex Administration > Licensing, a read banner stating ". . . license not imported because license is invalid" appears and the license fails to import.
Ensure that your Nintex Databases are up to date.
For Nintex Workflow Databases: -For SharePoint 2016: Navigate to Central Administration > Nintex Administration > Database Management and check if there is a link to upgrade your database. -Alternatively, you can force a database upgrade using the following command from the SharePoint Management Shell:
NWAdmin.exe -o UpgradeDatabases
For Nintex Forms Databases, use the following two commands from the SharePoint Management Shell: Install-NFService Upgrade-NFService
If this issue occurs in SharePoint 2010 or SharePoint 2013 the troubleshooting steps would still be to ensure that the databases are updated, however, the methods to do so may differ.