I am trying to activate a Vuforia License in my unity project, But when i paste my license key in the "App license key " bar the "Add license " button disappears.
could someone explain me how to fix this issue
The Add License button is there to quickly open the Vuforia Developer Portal page to allow you to create a license key if you don't have one yet.
After you create a license key, copy-paste it into the VuforiaConfiguration's App License Key field and your project will be ready to use all the Vuforia features included in your license. No further step is needed to activate the license.
If you encounter any issue, please feel free to create a new Issue in Vuforia's public GitHub repository: https://github.com/PTCInc/vuforia-engine/issues.
Additionally, it is strongly recommended to not share your license key publicly. I would suggest you to delete the link to the picture in your question and create a new one before you continue developing with Vuforia Engine.