You are most likely combining the wrong library version with your license key. If you bought an iText 5 license, please use version 1.x.y. If you bough an iText 7 license, please use 3.x.y.
This can mean a few things:
The license key file was corrupted during the download. Try downloading it again and loading the newly downloaded file. If the issue still persists, contact us so that we can resolve this issue.
You changed the contents of your license key. Please revert to the original state of the license key. If you want to change the content of the key, please contact our sales department.
You are trying to use an add on product without having loaded a license key that contains a valid license for that product. If you did buy a license for the add-on, please contact our sales department to rectify this. If you didn't buy a license and you want to try out the add on, you may want to register for a trial license.
You are most likely trying to load an additional license key containing new add ons. This can only work if the core information is the same. If it's not the same and it should be, contact us to rectify this issue. If it's not the same, please only use your own license key and don't pass your license keys around.