You are dealing with functions belonging to the Quadrature-elements. Take a look at this for instance. The following code works fine for me
sig_n1 = as_3D_tensor(sig)
s1 = dev(sig_n1)
Vs = FunctionSpace(mesh, "DG", 1)
sig_eq1 = local_project(sqrt(3/2.*inner(s1, s1)), Vs)
# For Plotting
import matplotlib.pyplot as plt
p = plot(sig_eq1, mode='color')
plt.colorbar(p)
plt.title(r"Von Mises",fontsize=26)
plt.show()
