Plot[{2/3, x, x^2, x^3}, {x, 0, 1}, 
    PlotStyle -> {{Dashed, Black}, {Thick, Orange}, {Thick, Red}, {Thick,
    Blue}}, AspectRatio -> 1/1, PlotRange -> {{0, 1}, {1/3, 1}}, 
    FrameStyle -> Directive[Black, 16], 
    PlotLegends -> 
    Placed[LineLegend[{None, "No Protocol", "Repetition Protocol", 
    "Feedback Protocol"}, 
    LegendFunction -> (Framed[#, FrameMargins -> 0, 
    Background -> White] &)], {Center, Top}], Frame -> True, 
    FrameLabel -> {{Style[#, Black, Bold, 16] &@("F"), 
    None}, {Style[#, Black, Bold, 
    16] &@("\!\(\*SubscriptBox[\(p\), \(1\)]\)"), None}} ]