Select either J2000 or B1950 reference frame. Suppress output when body below horizon will output ephemeris data only when the target body if above the horizon. These are only useful for specialized computer post-processing of ephemeris data. See the Horizons Documentation for a list of keys. If there was a problem with any of your inputs or settings, you will get an error message instead of an ephemeris.
Most error messages are self explanatory.
Here is what you should do. You should now see the main form showing the new setting. You should now see the main form showing the new setting. You should now see the main form showing the new setting. See documentation for list of abbreviations.