Hi JettyChang,
thank you very much for that hint! Actually, I have two Linux_for_Tegra folders:
ubuntu@ubuntu:~$ sudo find . -name nv_boot_control.conf
./Downloads/Linux_for_Tegra/bootloader/nv_boot_control.conf
./nvidia/nvidia_sdk/JetPack_4.5.1_Linux_JETSON_NANO/Linux_for_Tegra/rootfs/etc/nv_boot_control.conf
So, I tried to execute the command in the other location, and now it seems to run :-)