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you forgot to do

var('t')

you forgot to do

var('t')

sage: a = pi/4;
sage: x = cos(a)^3;
sage: y = sin(a)^3;
sage: z = cos(2*a);
sage: pl2=parametric_plot3d( (x + t*(derivative(x)), y + t * (derivative(y)), z + t * (derivative(z))), (t,0,1), texture="red");

you forgot to do

var('t')

You also need to re-so assignments of x, y, and z after you set a to a particular value. I.e.

sage: a = pi/4;
sage: x = cos(a)^3;
sage: y = sin(a)^3;
sage: z = cos(2*a);
sage: pl2=parametric_plot3d( (x + t*(derivative(x)), y + t * (derivative(y)), z + t * (derivative(z))), (t,0,1), texture="red");

works just fine.