When working with projects in Cubase, sometimes a recorded file has to be deleted at the user's discretion (new audio files are created every time the record and stop buttons are used).
Deleting files from the Cubase's main window may erase them from the computer, but a reference of that file will still exist inside Cubase (think like having the recycling bin in Windows).
When loading back the project associated with these files, an error window will pop up.
To clear these errors, since the files are not needed any longer, in Cubase's main window, go to the Media tab and Open Pool Window option.
The Pool menu will open and it will list all the files in the project. You can see the “missing files” in the Trash tab. You can erase them here.
Alternatively, in the same Media tab, there's the option to clear them directly by using the Remove Missing Files option.
This is how the Pool window looks like when there are no missing files.