iPhone 5s stuck in recovery mode : Error 50

I would first try to force the phone into DFU mode and try again making sure the cable is original, the battery is holding some charge like 25% or more, and iTunes is updated to the last version.

If you have previous liquid damage or other physical damage that affected your board, it may fail all the restore attempts until the damage is fixed - if repairable.

To force the phone into DFU mode: connect it to the computer, with iTunes turned on.

Press & hold Power Button for 2 seconds;

After 2 seconds, keep pressing on the Power Button and simultaneously press & hold the Home Button (for iPhone 7/7+, the Volume Down button);

After 12 seconds, release Power Button, keep pressing Home Button (for iPhone 7/7+, the Volume Down button);

After maximum 15 seconds, release the Home Button (for iPhone 7/7+, the Volume Down button). iTunes on your computer must be by then showing a prompt saying an iPhone/iDevice in recovery mode has been detected. The screen on your iPhone/iDevice must be black with nothing showing on it. Otherwise you are in "Restore Mode" not in DFU mode, and should repeat the steps above.

Once in DFU mode, click on the option "Restore iPhone/iDevice" that you must be seeing as an option in iTunes. Leave it alone until it restarts on its own.

It may take up to a few hours to download the firmware from Apple, if your Internet connection is slow.

Once done, all your data will have been erased.

If you wish to (try and) retain your data, you may try a slightly different approach:

Turn off the iPhone/iDevice. Press & hold the Home Button (for iPhone 7/7+, the Volume Down button) and while holding, connect to your computer using your USB cable. The iPhone/iDevice will restart. Keep holding the button till it has restarted and showing "connect to iTunes" or the USB cable image on your screen. At the same time, iTunes on your computer should be saying that an "iPhone in restore mode has been detected" and you have the option to "Update" or "Restore." Try Update and leave alone till it restarts on its own.

Hope this fixes it.