Why does my bob keep going backwards

Why does my bob keep going backwards

I just got a used Bobsweep pet hair. When I switch it on it keeps going backwards, showing trouble Nr 06. I have followed all the instructions on the Internet including cleaning, running a test, etc. I knew that it doesn't auto dock. 06 is the bumper. But there is nothing stuck inside. It does not seem to be ‘clicking' on the right side.