You are currently overwriting your loop counter variable (t). This is incremented automatically by the for loop. You don't need the last 2 lines of code in your for loop.
You are also overwriting your value for T_new everytime. Instead, use your loop counter to store each temperature value in a unique column or row of T_new.
I also don't think you need to do this symbolicaly. You can remove the syms t at the top as well.
BTW, to create a vector of times from 0 to 200 with a 0.01 step size, you could just do the following code: