forgot iPad password, trying to restore from mac, response was iTunes could not connect to this iPad, the value is missing.
How to restore old iPad if it was not connected to iTunes.
If you are having difficulties with your iPad being recognised by your computer, try using a different USB port. Otherwise, this support page may help with troubleshooting connection issues:
If your computer doesn't recognize your iPhone, iPad, or iPod - Apple Support
These pages may also be useful: