This can be also done interactively from JMP but it is faster for me to just write it as a script so here it is. Notice that have a bit different table than you do (different column names and values).
Names Default To Here(1);
dt = New Table("Untitled",
Add Rows(10),
Compress File When Saved(1),
New Column("Step", Character, "Nominal", Set Values({"W", "E", "P", "F", "S", "C", "W", "P", "S", "F"})),
New Column("S", Character, "Nominal", Set Values({"F", "F", "F", "F", "F", "S", "S", "S", "S", "S"})),
New Column("M", Character, "Nominal", Set Values({"", "F", "", "P", "", "", "S", "G", "", ""}))
);
wait(1); // demo purposes
dt << Select Where(Is Missing(:M)) << Delete Rows; // << Get Rows Where() / << Where() are generally faster than select where
wait(1); // demo purposes
Column(dt, "Step") << Set Each Value("S");
-Jarmo