Just overcome this problem. Key step is that you need to download the latest firmware version of the device onto your hard drive . With some reason iTunes seems to forget old models of iPad and iPhone. Restore button on iTunes is for recent models only.
With the right firmware version on your hard drive, before clicking restore button, you need to press and hold “Shift” then click “Restore iPad”. It will bring you to file explorer. You browse to location of the downloaded firmware. Then click the firmware file, and click OK. iTunes then will restore your device with your saved firmware. Be mindful that you iTunes will not allow a restore to lower version than your latest one. If the process fails, I suggest you may download another version and try again. Hope this helps.