You have
ldeqdiff = laplace(deq,t,s);
which assigns to a variable name that begins with a lower-case 'L'. Then you have
ldeqdiff=subs(1deqdiff)
which attempts to subs() on an expression that begins with the digit 1 not with a lower-case 'L'.
deq = 'c*D2I + d*DI + (1/e)*I';
ldeqdiff = laplace(deq,t,s);
laplace() has never been documented as accepting a character vector -- not even in the days when the Symbolic Toolbox was based upon Maple instead of MuPAD.
