Dear Sohail,
The error message indicates that the Edit Field's Value must be a character vector or string. MATLAB does not automatically convert a 'sym' variable to a char, but you can do so by using the "char" function.
So in your case, you need to change the line:
>> app.Time_EquationEditField.Value = iLap1;
to
>> app.Time_EquationEditField.Value = char(iLap1);
Hope this helps!
-Aiswarya