When one clicks the streaming button, that's when authentication should be made with Twitch. Not right after running OBS.
For why would OBS need to authenticate with Twitch when OBS is launched. Something not right in the program if this is the way it behaves. More so in the sense that if you just wanted to record.
You probably entered in the wrong information when setting up with Twitch and can't be authenticated (or Twitch itself has problems). But again, this shouldn't be happening when you launch OBS.
Go into your log folder at C:\Users\Username\AppData\Roaming\obs-studio\logs and look at the latest log. Read through it to see if it logged any errors that describe what is going wrong related to your authentication problem.
Don't know what else to tell you. I stream to Twitch and don't get this problem when I initially launch OBS. If this was a common problem, you'd see many Twitch streamers here reporting the same problem. So its something wrong on your end and its likely a setup problem.
How to solve will probably require you to uninstall OBS.