When you say quits, do you mean that when it polls steamCMD it immediately terminates the program and closes, or that it fails to upload and quits from steamCMD?
Is your modname.vdf file in your Steam\steamapps\common\ArkDevKit\ModTools\ModConfigs\ folder? If so;
open steamCMD and use
login <username> <password>
workshop_build_item <path to file>modname.vdf
And see if it’s an error with your account, an error with the modname.vdf or the usual devkit steam upload issues.