On my system it works fine. The error message is probably from a conflict between your current assignement for Ampl and Equation and some other assignments that you gave earlier in the session and that is still in memory. Try with
Clear[Ampl,Equation];
Ampl[t_] := A*Exp[Alpha*t]
Equation[t_] := -5*Exp[-4*Ampl[t]] + 3*Ampl[t] - 4*Ampl[t]^2 + 2
