I think I may have found a bug.
On my store when accessing the cart, the order would already be loaded and used to view different line items. Then you can choose to checkout and be sent to a square auto generated checkout page. This is of course done using the payment link api to generate link.
This worked.
I then implemented a calculation step every time the cart is accessed. This works fine but when generating link to create checkout page ( the same flow ) it errors out.
applicationId: sandbox-sq0idb-Ml8VuI3UAOZaoZWw64Z05w
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Additional Documentation
Checkout API
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Configure the Order-Ahead Sample Application
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Actually upon further testing, I removed the calculation step and it seems the payment link still is not working, I am completely lost now. As I don’t know what else I have changed.
I took a look at your application and I don’t see any error from CreatePaymentLink. I checked both production and sandbox. What’s the error message your getting from Square? 
Ahh sorry I found the error, all due to my clumsiness. Before calling the payment link, I made a call to update the order to add services charges, that call errored out because I forgot to include version number.
