Dear Sungwoo Yang,
I cannot verify any error. I would like to propose some further changes. In particular below I replaced in your definition of the functions receff and syseff "=" with ":=", which might be important. So your already nice code together with Davids improvements and my minor remarks now looks like:
ClearAll["Global`*"]
\[Sigma] = 5.67*^-8; (* Stefan-Boltzmann Constant*)
\[Epsilon] = 1; (* Emissivity *)
\[Alpha] = 1; (* Absorbtivity *)
\[Tau] = 1; (* Transmittance *)
g = 1*^3; (* Solar radiation 1Kw/m^2" *)
c = Range[10, 50, 10]; (* Concentration *)
receff[x_] := \[Alpha] \[Tau] - (\[Sigma] \[Epsilon] (x^4 - 293.15^4))/(c g);
syseff[x_] := (\[Alpha] \[Tau] - (\[Sigma] \[Epsilon] (x^4 - 293.15^4))/(c g)) (1 - 293.15/x);
Plot[Evaluate[{receff[x], syseff[x]}], {x, 293.15, 1073.15},
PlotStyle -> (Directive @@@ Transpose[{Join[ConstantArray[Dashed, 5], ConstantArray[Thick, 5]],
Flatten[ConstantArray[{Black, Red, Blue, Green, Purple}, 2]]}]),
Frame -> True, PlotRange -> {{250, 1000}, {0, 1.05}},
FrameLabel -> {"Temperature (K)", "Efficiency (%)"},
LabelStyle -> Directive[Black, Bold, {16, GrayLevel[0.3]}]]
Henrik