how to erase and restore ipad when it doesn't have room and is locked

When your iPad is disabled, you need to completely restore it. It won't download the update over wi-fi when the device is disabled. You need to follow the steps here to restore your device to factory, then you can restore your latest backup. That will automatically install the latest update. If you forgot your iPad passcode - Apple Support

Use a computer to do so, if possible, making sure you're following these instructions - Erase iPad - Apple Support

If it continues to fail, try putting the device in Recovery Mode, then erasing it as shown here - If you can't update or restore your iPad - Apple Support