To use your licensed features, you simply log into the TeamViewer client on your computer with that same account. TeamViewer 2. Licensing Tiers
Your license has a device limit, and it is already maxed out. Solution: Deactivate an old, unused device from the Management Console, then retry activation.
You can confirm if your device or account is correctly licensed through these methods: Where do I find my license key? - TeamViewer Community