In OBS, go to Settings > Output , and ensure your bitrate is set between 4,000 and 6,000 kbps, or a maximum of 8,000 for 1080p 60fps.

What are you using (OBS, Streamlabs, etc.)? Are you connected via WiFi or Ethernet ? What is your current bitrate setting? How To Fix Twitch Showing OFFLINE While Your Streaming

Here is a feature breakdown of why this happens and how to fix it:

Use the Twitch Inspector tool to check for quality issues (aim for 100% quality).