1 please do the all four steps, please refer to QuickStart Guide — DeepStream DeepStream Version: 5.0 documentation, can you get nvidia-l4t-gstreamer package?, like this:
apt-cache search nvidia-l4t-gstreamer
nvidia-l4t-gstreamer - NVIDIA GST Application files
2 or you can use the newer version deepstream6.0.1,
Quickstart Guide — DeepStream 6.1.1 Release documentation