Error Please enter a username on Simplified registration with password

-- url of the page with the problem -- : mcnew.joomlademos.co.uk/index.php?option...ep&step=1&Itemid=444
-- HikaShop version -- : 3.0.0
-- Joomla version -- : 3.6.5
-- PHP version -- : 5.6.30
-- Error-message(debug-mod must be tuned on) -- : Error
Please enter a username.

I am having an issue, when users are trying to register or checkout as guest an error appears saying 'Error Please enter a username.'

I have checked everything on the site and all appears to be ok. Any ideas please?