Sorry for the late reply. I was able to recreate this problem with my copy after I making some mistake in the SHA-OUT parameters in the plugin configuration page. This is crucial that you know what the parameters are and what must be their values. Though your transaction is verified in the Barclay, they send some information back so that woocommerce can understand the status of the payment. If there is any misconfiguration with the SHA-OUT parameters then the returned value will not pass the verification in your site. So that the "Transaction verification error" will show up.
For more clarity, Barclay return a key value paired url encoded data and a hash code digesting those data as well. If the hash generated by digesting in your end doesn't match with the returned one, woocommerce will assume that there is an error with the verification of the transaction. So that you need to go thorough with those parameters.
There is bit confusion about the AAVCHECK and CVCCHECK parameters of Barclay. While I was developing this plugin they caused me a nightmare so that you should check them carefully.
Thanks a lot for using this plugin.