I had the exact same problem and I still don't know why this happens. I was able to fix it though by first unplugging the phone, then enter the command and then plugging phone back in and it should send right away. The problem may be the computer you could try another one. In my case I was not able to install lineagos so easily because the phone was always encrypted and I could never access the zip file in twrp. I don't remember what exactly was the problem but I tried a lot of times and it wouldn't work. My solution for this was to install everything via adb sideload command and twrp sideload function. Hope this helps!