@Ooops @FunnyDuck plenty of people have flashed GrapheneOS on their tablets, so it's not an issue with the web installer. 99 times out of 100, the issue is with one of the following: usb cable, computer port, the person doing the flashing is touching or moving either the computer or the device being flashed.
You can try multiple times. It's very hard to brick a Pixel device. If flashing doesn't work, try a different port, try a different device to flash from, or try a different cable.