Have you checked the file path listed in the error to confirm if the file actually exists there? I don’t use Dropbox. A quick online search shows it is a cloud based storage and backup service. Is this path to the file possibly a symbolic link to the online location, and not actually the file on your local disk? If so, then it would no longer work if your internet connection was disrupted.
Note that after you have transferred your geometry to Mechanical, it doesn’t matter anymore if the project can find the geometry, unless you want to make changes to the geometry in SpaceClaim or DesignModeler and refresh the geometry in Mechanical.
Does it recognize the geometry file when you make a new analysis system, specify the step file on the Geometry cell and “Edit” the Model cell?
It’s possible there is a character in the path it doesn’t like. Is that a vertical bar after the “I” or a capital I?:
C:\Users\Utente\Dropbox\I| mio PC (DESKTOP-HP545G0)\Desktop\mode_superposition\beam.STEP
Have you tried “View > Files” then right click on the red row to “Repair” and browse to the file?