If you have not any host PC with ubuntu installed, I’m afraid you can train on Cloud, such as GCP or AWS.
For how to set up GCP or AWS, please search similar topics in the TLT forum. Such as, Train instance segmentation model in google cloud instance with custom data and run inference on Jetson AGX Xavier with TensorRT
TLT 3.0 user guide describes the steps at
https://docs.nvidia.com/metropolis/TLT/tlt-user-guide/text/requirements_and_installation.html
https://docs.nvidia.com/metropolis/TLT/tlt-user-guide/text/tlt_launcher.html
https://docs.nvidia.com/metropolis/TLT/tlt-user-guide/text/tlt_cv_inf_pipeline/requirements_and_installation.html