Hey Michelle,
That license key field in your screenshot is under the Event Aggregator label. Events Calendar Pro is not Event Aggregator, and so entering your Events Calendar Pro license key in the Event Aggregator license key field will not work.
Please install Events Calendar Pro on your site.
Then activate the plugin.
Then go to Events → Settings. If you have installed and activated Events Calendar Pro, there should be a new “Licenses” tab there in the settings screen.
Go to that tab.
You’ll see an Events Calendar Pro license key field where you can enter your Events Calendar Pro license key.
Let us know if this helps! 😀
— George