You are attempting to run the program after you have
unloaded the main module. Every program must have a main
module so QuickBASIC knows where to start running the
program. Either
  - Use the Load File command from the File menu to
    reload the main module you unloaded. Or,
  - Use the Set Main Module command from the Run menu
    to assign the role of "main module" to one of the
    currently loaded modules.