I just put your command into a SQL Server Agent Jobstep configured to use CmdExec; it ran the command perfectly.
USE [msdb]
GO
/****** Object: Job [CmdExecTest] Script Date: 2018-03-19 10:55:07 AM ******/
BEGIN TRANSACTION
DECLARE @ReturnCode INT
SELECT @ReturnCode = 0
/****** Object: JobCategory [[Uncategorized (Local)]] Script Date: 2018-03-19 10:55:07 AM ******/
IF NOT EXISTS (SELECT name FROM msdb.dbo.syscategories WHERE name=N'[Uncategorized (Local)]' AND category_class=1)
BEGIN
EXEC @ReturnCode = msdb.dbo.sp_add_category @class=N'JOB', @type=N'LOCAL', @name=N'[Uncategorized (Local)]'
IF (@@ERROR <> 0 OR @ReturnCode <> 0) GOTO QuitWithRollback
END
DECLARE @jobId BINARY(16)
EXEC @ReturnCode = msdb.dbo.sp_add_job @job_name=N'CmdExecTest',
@enabled=1,
@notify_level_eventlog=0,
@notify_level_email=0,
@notify_level_netsend=0,
@notify_level_page=0,
@delete_level=0,
@description=N'This job provides a way to test SQL Server Agent Jobs',
@category_name=N'[Uncategorized (Local)]',
@owner_login_name=N'<me>', @job_id = @jobId OUTPUT
IF (@@ERROR <> 0 OR @ReturnCode <> 0) GOTO QuitWithRollback
/****** Object: Step [Step1] Script Date: 2018-03-19 10:55:07 AM ******/
EXEC @ReturnCode = msdb.dbo.sp_add_jobstep @job_id=@jobId, @step_name=N'Step1',
@step_id=1,
@cmdexec_success_code=0,
@on_success_action=1,
@on_success_step_id=0,
@on_fail_action=2,
@on_fail_step_id=0,
@retry_attempts=0,
@retry_interval=0,
@os_run_priority=0, @subsystem=N'CmdExec',
@command=N'sqlcmd -E -S $(ESCAPE_SQUOTE(SRVR)) -d master -Q "EXECUTE [dbo].[DatabaseBackup] @Databases = ''USER_DATABASES'', @Directory = N''C:\Backup'', @BackupType = ''DIFF'', @Verify = ''Y'', @CleanupTime = NULL, @CheckSum = ''Y'', @LogToTable = ''Y''" -b',
@flags=32
IF (@@ERROR <> 0 OR @ReturnCode <> 0) GOTO QuitWithRollback
EXEC @ReturnCode = msdb.dbo.sp_update_job @job_id = @jobId, @start_step_id = 1
IF (@@ERROR <> 0 OR @ReturnCode <> 0) GOTO QuitWithRollback
EXEC @ReturnCode = msdb.dbo.sp_add_jobserver @job_id = @jobId, @server_name = N'(local)'
IF (@@ERROR <> 0 OR @ReturnCode <> 0) GOTO QuitWithRollback
COMMIT TRANSACTION
GOTO EndSave
QuitWithRollback:
IF (@@TRANCOUNT > 0) ROLLBACK TRANSACTION
EndSave:
GO
The result was a job failure for me because I don't have the dbo.DatabaseBackup
stored procedure:
Msg 2812, Level 16, State 62, Server MyServer, Line 1
Could not find stored procedure 'dbo.DatabaseBackup'.
However, this indicates that sqlcmd was successfully executed.