In your code, check with the symsum function as it do not return the sum of the series((-D*n^2*pi^2*t)/(r^2))) with respect to the summation index n. You can use the subs function to get the actual array of Dsol values as shown below.
Here, t1 = [0.50373, 43.2325, 86.72428, 129.74978, 173.8774, 216.69095]
M1 = [0.75233, 62.53212, 67.84044, 67.6522, 80.90417, 82.59832] and
r1 = 130/(10^9);
For more information, refer Substitute Multiple Scalars with Arrays from this link