The optimization of atomic coordinates can be done by calling "OptimizeGeometry()" with "disable_stress=True", e.g.,
new_configuration = OptimizeGeometry(
bulk_configuration,
maximum_forces=0.05*eV/Ang,
disable_stress=True,
trajectory_filename=None,
)
If the CNT in your supercell is built along the x direction, your setup for k-point (12*2*2) may be OK. But if it is not, e.g., the periodic length of CNT along the z direction, you should change the setup for k-point into 2*2*12.