Your command looks correct so far. Are you sure that the path to the TWRP image was correct? Also could you post the output of fastboot oem device-info?
Also, you really really (really) shouldn't flash twrp-3.2.3-1-NB1.img. It has no ability to boot into your system and instead boots straight into recovery. It was intended to be used as an image you could temporarily boot and then patch your local partition with a special installer .zip, but when fastboot boot broke it became useless.
Either use the prepatched image that matches your security patch, or my new generic TWRP image that works on all security patch versions (and can decrypt /data on Android 9). You can find both at https://bit.ly/nokia-nb1
Your generic TWRP works on Oreo & on Pie too, yes??
Also, just to make doubly sure, command should be " fastboot flash boot_a twrp-3.2.3-2-NB1.img " followed by " fastboot reboot ", and that should be it right...??