Hi,
Looking at the code, I can see that the error message is not handled properly.
Please change the line:
JFactory::getApplication()->enqueueMessage('Sorry your transaction did not go through successfully, please try again.<br/>Error:'.$invoice['error']['message'], 'error');
to:
JFactory::getApplication()->enqueueMessage('Sorry your transaction did not go through successfully, please try again.<br/>Error:'.$invoice['error'], 'error');
in the file plugins/hikashoppayment/hikabitcoin/hikabitcoin.php and try again.
You should then get the end of the error message which will explain what is wrong in more details.