“Linux_for_Tegra/” is a subdirectory, and “flash.sh” exists in that subdirectory. If you have installed host side JetPack/SDK Manager, go to “~/nvidia/nvidia_sdk/”. From there there are JetPack directories named after the particular board. Assuming JetPack4.3, look at:
~/nvidia/nvidia_sdk/JetPack_4.3_Linux_<designation for nano>/Linux_for_Tegra/
I have not used “jetson-image-disk-creator.sh”, so I can’t really answer anything about it. If you want to see which JetPack/SDK Manager releases are available, then go here (you may need to go there, log in, and then go there again if redirect fails):
JetPack4.3 installs L4T R32.3.1 and is the most recent (this is recommended).