Takepayments addon not working properly

Site URL: https://www.easy-irrigation.co.uk/
ZenCart version: v1.5.5a
PHP version: 7+ (can't remember exactly)

I'm attempting to setup the "takepayments" addon provided by https://www.takepayments.com/developer-support/shopping-carts/ but it is not working as expected and I can't seem to find any proper support on it.

The module is on my site but whenever I select the option at checkout I just get the following error:
"Sorry but we could not verify this transaction. Please refer this error the merchant or administrator"

Nothing really descriptive and I've tried to get more descriptive errors but have failed. I've also followed the instructions for setting up the plugin but something isn't working here.

If any more information is needed please let me know! This is my first time posting