“/dev/ttyACM0” is not a real file (it exists in RAM only). This is a device special file, and is the result of the driver being loaded and pretending to be a file. Add the tty-acm driver mentioned by @Kangalow, and the file will appear.
One method to get the driver is mentioned, that of building it from the kernel source. Official docs for your L4T release explain how to do this in the kernel customization section. It isn’t particularly difficult, but it sounds intimidating until you’ve done this once. The main thing is to configure the source to the existing configuration, which is by default the “tegra_defconfig” build target, combined with setting “CONFIG_LOCALVERSION” to “-tegra”. After that you’d build the “modules_prepare” target to propagate the config, and then it is a simple file copy from the built file to the module directory of the Jetson.