Please run below to check if the path is available.
$ tlt detectnet_v2 run ls /workspace/tlt-experiments/detectnet_v2/specs/detectnet_v2_tfrecords_kitti_trainval.txt
If not, there should be something wrong in your ~/.tlt_mounts.json file.
You can login the docker and narrow down and debug.
$ tlt detectnet_v2
#
ls /workspace/tlt-experiments/detectnet_v2/specs/detectnet_v2_tfrecords_kitti_trainval.txt
See TLT 3 error running detectnet_v2 dataset_convert for reference too.