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You just reassigned d too early. d needs to be a variable to do anything with it, and your determinant now has a variable that isn't a variable - d refers to .5, but the symbolic expression doesn't know you did this.

Change the last bit to

f(a,b,c)=A.determinant()
implicit_plot3d(f(d=.5), (a, 0,1), (b, 0,1), (c, 0,1),plot_points=100)

and you should get a pleasing graphic (the plot_points because otherwise it's a bit pixelly at some singularities of the surface. Cool stuff!