While configuring the license, you might see not a valid license string message. The Acrolinx Connector license for confluence can be configured from the ⚙️ → Manage Apps → Acrolinx Connector License menu.
Upon entering your license, if you see a not valid message like above, please check that you are using the correct license.
The Acrolinx Connector License is always hexadecimal (customer specific), do not mix it up with the Atlassian License. On entering the correct license, you should be able to see the Organization, Created at and Expiration fields populated like below.