So, After a little digging, it's seems to be that, it's Vue.js problem. the software is made by Laravel(Backend) and Vue.js (Frontend).
Can you create a ticket and ask them about issues, - https://pay.thetshirtman.co.uk/invoices
Error file - resources/js/Pages/Invoice/List.vue
Please use a proper editor or IDE. Those are components.
Can you please let me know what the problem is on your site?
That's not due to the file as those are already bundled. Please check error logs in the
storage/logs/ folder and let me know the error.
If you are not sure, please share the server access details in a private reply. I will help you check.
You can add a private reply by enabling the
PRIVATE for Staff only at top right of reply section.