There is no function called ‘T’ so ‘T(0)’ is not defined.
Try this instead —
syms I(t) Mu To t %Used to decalre variables as symbol
ode = diff(I,t) == -Mu.*I(t); %Equation Writing
cond = I(0) == To; %Initial condition
Solution_eq2 = dsolve(ode, cond) %Solution the the modified equation == Solution to the IVP
.
