Terminal Pop Up Window

That looks like something in your profile or other shell startup file is trying to change the shell to zsh. You need to look at Terminal Settings > General tab > Shell opens with: or Profiles > Default Profile > Shell tab > Startup.

If it is not there, it is probably in ~/.zprofile or ~/.zshrc.

Since it pops up on startup, I would think some app is running a background process via Terminal. Your login config documents might be interrupting that.

If the Terminal app is launching automatically, then check your Login items to see if it is listed there. Otherwise, some third party software is probably utilizing the Terminal and that third party software is not behaving properly. If this is the problem, then run EtreCheck and post the report here so we can examine it for clues....not sure it will help us identify the offending app, but we may notice some third party apps which have a bad reputation here on these forums.

You can try booting into Safe Mode to see if you encounter the same issue. If the issue does not occur in Safe Mode, then it confirms the issue is most likely the result of some third party software not behaving correctly.

If you can provide a screenshot of the Terminal window which appears, it may help us to identify the problem.

This tells us that the chsh command was inserted in one of the files that are read when your active shell starts.

This could be ~/.bash_profile if it is bash.

If you want to switch to using zsh, which is currently the default recommended shell, it is perfectly safe to then type your password as requested. Then I would quit Terminal and start it again, and see if the same thing appears (probably not).

If you are using bash and do not want to change, then let us know, and we can try to guide you to edit the relevant file and remove the call to the chsh program.

You failed to post the image, but from your description this is just an information that the default shell is now zsh instead of bash. There is no problem if you use bash, though. You can safely ignore this, or you can change the shell to zsh if you want.

It is usually polite to post the solution here so we can learn from it as well as helping anyone else who happens upon your thread seeking the same assistance, or would you like them to have to stumble blindly knowing someone figured it out, but didn't provide the solution.