write error: Invalid argument using linux remoteproc on BeagleBone Black

Question

I have been trying to further understand the Sitara Am335x PRUs and I tried to follow the TI labs on how to get started with Programmable real-time units on the BeagleBone Black and but I encountred this problem that I couldn't overcome;

sudo -i  
cp /home/debian/pru_blinky.out /lib/firmware/pru_blinky
cd /sys/class/remoteproc/remoteproc1  
echo 'pru_blinky' > firmware

but when I type echo 'start' > state I get the following error

this is the return of man echo

NAME
   echo - display a line of text

SYNOPSIS
   echo [SHORT-OPTION]... [STRING]...
   echo LONG-OPTION

DESCRIPTION
   Echo the STRING(s) to standard output.

   -n     do not output the trailing newline

   -e     enable interpretation of backslash escapes

   -E     disable interpretation of backslash escapes (default)

   --help display this help and exit

   --version
          output version information and exit



-bash: echo: write error: Invalid argument

I tried to to specify the directory and file with command: echo 'pru_blinky/pru_blinky.out' > firmware but still problem not solved can anyone explain to me what am i doing wrong here