Hi,
According to the Nu Vision website support area for your tablet, it says in response to question No.13:
''Can I '''upgrade''' the processor or '''Android operating system'''?''
'''''No'''. Any attempt to upgrade the hardware or change the operating system on the tablet may void the Warranty and may compromise the operation of the tablet.''
Here is the link where the above information can be found. The information is relevant to ALL Nu Vision tablets.
http://www.nuvision.com/faq
As there are ''no updates available at all, from the Nu Vision website ''it supports the above statement.
Did you manually try to update the tab? If so that is why you are getting the unknown error message.
They must have 'customized' the OS to suit their requirements and in doing so have disabled or removed a part of the system update procedure for the reason that they do not want the OS updated from its' present version. They should really have removed the system update option or grayed it out so that users could not see or use the option at all but that is their concern.