It has lots of reason to cause such problem. Honestly, It is not an easy one to debug.
ttyACM0 requires
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The device side is running nv-l4t-usb-device-mode.service without problem.
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The usb cable is good enough
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The host side has the driver to support this interface.
You have to make sure each of them is fine. Honestly, most users have problem in point 3 because their host does not have those driver supported. Also, so far only ubuntu host supports it.