This message indicates that either your free trial period has expired, or that your product license was deactivated.
Many of our products have fully-functional trial periods, and once this trial period is over, you’ll need to purchase a license to continue to use it. “Perfectly Clear” will continue to be usable forever in ‘Free Forever’ mode.
Deactivation can happen if you manually deactivate (so you can use that license on a different computer). If that license is not in use on other computers, you can just click the menu button in the top right, then click ‘Activate’ – it should remember your previous license key.