The error/warning has occurred as “dsolve” was unable to find a symbolic solution for the given differential equation.
As a workaround, a numerical solution can be obtained by using “ode45”. Here is an example code to solve the equation numerically for 
.
.% Define the system as first-order equations
% Let y1 = y, y2 = y'
% Then, y1' = y2, y2' = -y1
f = @(t, Y) [Y(2); -Y(1)];
% Initial conditions
Y0 = [1; 0]; % y(0) = 1, y'(0) = 0
% Time span
tspan = [0 10];
% Solve using ode45
[t, Y] = ode45(f, tspan, Y0);
For more information on “dsolve” and ”ode45” you can type the following commands in a MATLAB command window.
doc dsolve
doc ode45
Hope this helps.
