Dear Sir,
you have purchased on Feb 25, 2020, and it was a QuickOPC license without any "upgrade assurance" (which is offered for 1/2/3/4/5 years together with initial purchase).

This means that the license only covers the version that was current at the time of purchase, earlier versions, and versions released in 60 days following the purchase, which comes to Apr 26, 2020.

According to the version table (kb.opclabs.com/Versions ), the latest version you can use with your license key is thus QuickOPC 2020.2.

You are currently running a later version (2020.3), and that's why your license key is not recognized.

You can either revert to using version 2020.2 or earlier, or purchase version upgrade.

Best regards