Talkback is not getting enabled through adb shell

I've tried the command adb shell settings put secure enabled_accessibility_services com.android.talkback/com.google.android.marvin.talkback.TalkBackService to enable talkback from adb shell. It's toggling the ui button which signifies the status of talkback, but talkback is not actually getting enabled. I am trying to enable talkback programmatically for android 6.0. Is there any other way thorough which I can enable it?

How fix adb inaccessible or not found?

Run the command without adb shell prefix, just like this: am start -n <...> Or you can do exit and then type the original command: exit adb shell am start -n <...>

Commands to toggle TalkBack:

// disable
adb shell settings put secure enabled_accessibility_services com.android.talkback/com.google.android.marvin.talkback.TalkBackService

// enable
adb shell settings put secure enabled_accessibility_services com.google.android.marvin.talkback/com.google.android.marvin.talkback.TalkBackService

Kudos to @zeh