Difficult to say without the full code. You are using numerical integration so, you will have to define the parameters before integrating. This here, for example, works:
g = -9.81; a = 0.4; ve = 4.3; mr = 0.2; mw = 0.5; b = 0.1;
V = NDSolve[{v'[
t] == -g + (a*ve)/(mr +
mw*Exp[-ve*t]) + (mw*ve*Exp[-ve*t])/(mr + mw*Exp[-ve*t])*
v[t] - (b/(mr + mw*Exp[-ve*t]))*v[t]^2, v[0] == 0.01},
v[t], {t, 0.01, 0.4}]
Plot[v[t] /. V, {t, 0.01, 0.4}]
Cheers,
Marco
