Why does Quicktime Player fail to start

QuickTime Player started normally though. What you are seeing, is that QuickTime Player defaults to offering to open a compatible document (video or audio clip); the standard Open-dialog box, as most applications have.

Normally it will remember the last used location, so if you were to open a document on the Desktop now, then it will default to that location on the next use.

If you do not want to open an existing QuickTime-compatible document at this time (but wanted to use a different function, like making a recording), then click Cancel, and then choose a different option from the menu bar.

Be careful where you click though, as QuickTime Player may quit automatically when it has no windows open and you switch between apps (including Finder).

BUT the document QuickTime "offers" is greyed out — see top Window in my attached screenshot.

In the initial screenshot, it currently points to the Application folder. None of the apps in the Applications folder can be opened by QuickTime Player. You’d have to navigate to a video file or audio file. It would gray out incompatible items in the list. The sidebar of the Open window should allow for shortcuts to where you want to navigate to, to find your file of choice. Else you can even drag-and-drop a file or folder into the Open dialog box, to change its focus.

Can you navigate to a folder where there are movies and open them?