I have installed the MarketPress plugin and it won't

I have installed the MarketPress plugin and it won't complete the sale. Every time I click the Continue button it display an error 10413.

I've tried deactivating the PayPal Express Checkout payment option and use only Stripe. This has a spinner for processing and then nothing. It stays on the Checkout page.

I also noticed the total amount payable is wrong. £24.99 is the price I entered including taxes however for some reason it hasn't take this into account, plus the Tax amount is incorrect .

I've enabled support access to the website. It's on a test domain at the moment - http://www.thmwp.co.uk

Thanks

Andrew