Running a test order: "There was an issue processing your payment information"

Site URL: http://doublea.photography

Hello.

I'm trying to run a test order on my Squarespace website but I keep getting the following error message:
"There was an issue processing your payment information. You can return to PayPal and select a different funding source, or enter your credit card details." (see attached file)
I'm using my personal PayPal to login and pay for the test order (as stated on the Help section on Squarespace), and have already connected my PayPal business account to my Squarespace Business Plan website.
I have a bank account connected to my PayPal Business account.
I have tried running the test order in Incognito mode, and a friend has tried ordering, but I still get the same error message. I have contacted Squarespace and chatted with them for about an hour yesterday but they unfortunately couldn't resolve the issue, and asked me to contact PayPal. I am currently awaiting PayPal's response.

Please help!
I have temporarily disabled the store on my website.
/Arvind
(London, UK)