I had a quick look at the site linked to your license key and its loading over HTTP so it would show as ‘non-secure’. To prevent that you’ll need to enable HTTPS on yout site, do you have an SSL certificate for the site? Or, does you how support “Let’s Encrypt”?
The button did work for me, what happens when you try to select tickets?
The site is loading over HTTPS so it looks correct to me.
Something that does stand out is you are using PayPal Pro as a payment method, that means you will be taking on full PCI compliance responsibility yourself, just checking if that is something you are aware of?