If the Debugger is already loaded, you have not specified a filename after a READ command or after a command to redirect output to a file. Repeat the command, specifying the name of an input or output file.
If you were at the system prompt, you used the -F command-line option to invoke the Debugger, but you did not specify the name of an initialization file. Repeat the command, but this time either omit the -F option or specify the name of a .4db initialization file following the -F option.