Device tree overlay failed. Cannot overlay BB-BONE-LCD7-01 dtbo file.

Hello, everyone. I have a LCD7 RGB screen. I wanted to use it in my BB-Black.
I added “capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN” to the uEnv.txt file. Then I typed “echo BB-BONE-LCD7-01 > $SLOTS”, but BB-Black replied “-sh: echo: write error: No such file or directory” to me. I am sure the dtbo file exists. Anyone could me?

Come on, my friends. Plz help me.