I can't get rid of a yellow line! Please help?

Whenever the application behaves unexpectedly, the first step is usually to delete the preferences.

To delete preferences, hold down command and option while starting FCP X and click Delete Preferences.

The application will start in a default state, and open an "Untitled" library. Reopen the library you were working on, set the preferences the way you like them, and try again.

It looks like you activated a guide. Check the View menu in the upper right of the Viewer. The guide line shouldn’t export.

Sometimes there is a similar situation, but after restarting the application everything becomes normal.

A similar question appeared last year and resetting the preferences as Luis suggested cured it.

https://discussions.apple.com/thread/251621202

Tom, part of the OP issue is that said View menu is missing.

Thanks Luis.