Transaction error

Hi,

I am trying to use this plugin to connect to our Square account. I have installed and hooked up to our account and the sync appears to work.

However whenever I try and process a payment I get an error in the UI :

Error: Unable to complete your transaction with square due to some issue. For now you can try some other payment method or try again later.

If I check the log files in WP I can see an error :

02-18-2020 @ 18:58:32 - Info: Begin processing payment for order 21958 for the amount of 2.00
02-18-2020 @ 18:58:33 - Error: Array
(
    [0] => stdClass Object
        (
            [code] => VALUE_EMPTY
            [detail] => Field must not be blank
            [field] => source_id
            [category] => INVALID_REQUEST_ERROR
        )

)

Hope you can help.