1st Gen iPad Air iOS9 to iOS12 - Problems updating with iTunes

I am trying to update my first gen iPad Air from iOS 9.3.1 to iOS12 Because the jump is so large it requires I use iTunes and not OTA. This is OK...however, when I try to update I get an error message as follows:

There was a problem downloading the software for the iPad. You do not have permission to access the requested resource.

I have tried:

1) Wiping the device with erase all content and settings and setting up as new iPad

2) Turning of wi-fi on iPad

3) Using iTunes on a different laptop

Nothing is working and it is driving me insane! Has anyone encountered this problem? Any info would be most appreciated.