Disproportionation reactions: In these type of reactions same species gets reduced and oxidised simultaneously.
For a species to participate in a disproportionation reaction, it should be able to show variables oxidation state.
Fluorine is the most electronegative atom and has extremely small size.
Electronic configuration of fluorine:
1s22s22p5
Thus, fluorine readily accepts one electron to achive stable insert gas configuration. Since it is gaining one electron, it exists as F− ion with -1 oxidation number.
Due to its highest electronegativity, it cannot lose electron, that's why fluorine does not show variable state.