Because you did not assign values to the expressions.
kp = 4;
km = 5;
A[t_] := (c0/(1 + (km/kp)))*(Exp[-t (km + kp)] + (km/kp))
B[t_] := (c0/(1 + (km/kp)))*(1 - Exp[-t (km + kp)])
Plot[{ A[t]/c0, B[t]/c0}, {t, 0, 10}, PlotStyle -> {Blue, Red},
AxesLabel -> {"time", "Concentration"}, PlotRange -> All]