Exception when granting WRITE_SECURE_SETTINGS perm

[huemand]

 .\adb.exe shell "pm grant com.flx_apps.digitaldetox android.permission.WRITE_SECURE_SETTINGS"

Exception occurred while executing 'grant':
java.lang.SecurityException: grantRuntimePermission: Neither user 2000 nor current process has android.permission.GRANT_RUNTIME_PERMISSIONS.
        at android.app.ContextImpl.enforce(ContextImpl.java:2343)
        at android.app.ContextImpl.enforceCallingOrSelfPermission(ContextImpl.java:2371)
        at com.android.server.pm.permission.PermissionManagerServiceImpl.grantRuntimePermissionInternal(PermissionManagerServiceImpl.java:1360)
        at com.android.server.pm.permission.PermissionManagerServiceImpl.grantRuntimePermission(PermissionManagerServiceImpl.java:1342)
        at com.android.server.pm.permission.PermissionManagerService.grantRuntimePermission(PermissionManagerService.java:570)
        at android.permission.PermissionManager.grantRuntimePermission(PermissionManager.java:615)
        at com.android.server.pm.PackageManagerShellCommand.runGrantRevokePermission(PackageManagerShellCommand.java:2692)
        at com.android.server.pm.PackageManagerShellCommand.onCommand(PackageManagerShellCommand.java:291)
        at com.android.modules.utils.BasicShellCommandHandler.exec(BasicShellCommandHandler.java:97)
        at android.os.ShellCommand.exec(ShellCommand.java:38)
        at com.android.server.pm.PackageManagerService$IPackageManagerImpl.onShellCommand(PackageManagerService.java:6587)
        at android.os.Binder.shellCommand(Binder.java:1075)
        at android.os.Binder.onTransact(Binder.java:895)
        at android.content.pm.IPackageManager$Stub.onTransact(IPackageManager.java:4352)
        at com.android.server.pm.PackageManagerService$IPackageManagerImpl.onTransact(PackageManagerService.java:6571)
        at android.os.Binder.execTransactInternal(Binder.java:1351)
        at android.os.Binder.execTransact(Binder.java:1282)