[Support request] Stripe test card not working

Hi, I’m testing a Stripe Buy Now button on my website with sandbox mode following the instructions on this page: https://simple-membership-plugin.com/create-stripe-buy-now-button-for-membership-payment/

However, when I enter test visa card details and press the pay button, I get an error saying that a technical issue is occurring on the website.

Debug log only says the following and I can’t see any error here:
SUCCESS: Stripe Buy Now IPN received. Processing request…
SUCCESS: Sandbox payment mode is enabled. Using test API key details.

I created a similar PayPal Buy Now button but it also doesn’t work with an error saying: “Things don’t appear to be working at the moment. Please try again later.”

Does anyone have clue what this is?

SWPM version: 3.8.4
Wordpress version: 5.2.2