Array
(
[0] => Square\Models\Error Object
(
[category:Square\Models\Error:private] => AUTHENTICATION_ERROR
[code:Square\Models\Error:private] => UNAUTHORIZED
[detail:Square\Models\Error:private] => This request could not be authorized.
[field:Square\Models\Error:private] =>
)
)
Not sure if it has anything to do with the Redirect URL, please help
This error happens when either the credentials aren’t configured correctly or the environment that you’re calling doesn’t match the environment of the credentials. For example with productions credentials you’ll need to target the production environment. Same with Sandbox. 
I found out the error comes up when I use the Application Secret instead of Access Token. However, when the Access Token was used, there was no error but the payment interface is not displayed to accept payment.
Not sure I follow what’s going on. What do you mean that the payment interface isn’t displaying?
I expected the page for credit card details input to be displayed to allow customers to make payment.
My understanding is that a Square hosted payment page displaying input area for Credit card number, Expiry Date and CVV will be displayed when customers click the Payment button but this is not the case.
Please clarify, I might be getting something wrong here.
If you’re using the Checkout API to create payment links than you’d be redirected to a Square hosted checkout page where you enter the card information. If you’re using the Web Payments SDK then the form fields to enter the card information will render on the page itself without redirecting. You mentioned that you’re using WordPress. Are you using the supported integration or is this a custom integration?
I am using a supported integration VikSquareUp SquareUp integration to collect payments through the Vik plugins
I’m not seeing VikSquareUp listed in our App Marketplace which means that we doesn’t currently have a direct partnership with them. This isn’t a problem. I tried to find the documentation on how to configure it with Square but it looks like it’s not publicly available. I recommend reaching out to this company directly for help. They’ll know how to configure it with Square since they’re the ones that configured the application.