Integrate PayPal Checkout, code examples not working

[MetroDesignsUK]

I'm new to the Paypal Developer Environment with some coding knowledge.

I've been following the code examples on the following page:

https://developer.paypal.com/studio/checkout/standard/integrate 

I've added my client id and client secret to the .env file and uploaded the files to my server. I'm using my Sandbox credentials.

The buttons load ok, but when you click on one of them a popup appears and quickly disappears. I used a screen recorder to capture the message.

The code is live at www.metrodesigns.co.uk 

Any help would be much appreciated.