Show adb backup manual

That's because you need to connect a device to adb first. Check for connected devices, using adb devices.

Otherwise, it'll display an error:

C:\Users\User\AppData\Local\Android\Sdk\platform-tools>adb shell bu help

error: no devices/emulators found

When you've connected device, run adb shell bu help - it will output something similar:

C:\Users\User\AppData\Local\Android\Sdk\platform-tools>adb shell bu help

 backup [-f FILE] [-apk|-noapk] [-obb|-noobb] [-shared|-noshared] [-all]


  [-system|-nosystem] [-keyvalue|-nokeyvalue] [PACKAGE...]


  write an archive of the device's data to FILE [default=backup.adb]


  package list optional if -all/-shared are supplied


  -apk/-noapk: do/don't back up .apk files (default -noapk)


  -obb/-noobb: do/don't back up .obb files (default -noobb)


  -shared|-noshared: do/don't back up shared storage (default -noshared)


  -all: back up all installed applications


  -system|-nosystem: include system apps in -all (default -system)


  -keyvalue|-nokeyvalue: include apps that perform key/value backups.


(default -nokeyvalue)

 restore FILE

 restore device contents from FILE