Please try the following things that that should help resolve this issue:
Log out from SLOBS, restart the application as administrator and log back in.
If using Twitch ensure your not running a rerun of a previous stream as this will prevent you going live.
If you are simply end it and hit go live again.
Double check your stream key if this is still correct.
If using Advanced Output Mode and using NVENC/AMD make sure GPU is set to 0.
If signed into Streamlabs OBS with Twitch you can run the auto-optimizer found in General Settings.
I hope this method will help you solve your
OBS Streaming error.