Sandbox testing - use alternate form of payment

Hi, I am just trying to get my site up and sell some hot sauce.
I put my site in sandbox mode, and it worked beautifully the first day. Since then it has not worked at all. I get this error message when running any test card numbers:
An error occurred, please try again or try an alternate form of payment.
When I look at the log it says:
02-09-2021 @ 00:03:07 - Payment nonce is missing
02-09-2021 @ 00:07:26 - Square.js Response:
[0] => Array
[message] => Request not authorized.

Please help, I would really like to get this off the ground, but it seems like I wind up down rabbit hole after rabbit hole.

@sidnister welcome to the forums!

UNAUTHORIZED errors are due to invalid access tokens being used typically. On your backend, are you using the sandbox access token, along with the sandbox domain (…)?