BCP utility does not create file in SQL Server

Hi

I have an issue, I could not export data by using bcp command. I am using below code, but csv file cannot be created

Below is my code

declare @sql varchar(8000)
DECLARE @columnHeader VARCHAR(8000)
DECLARE @table_name  VARCHAR(50) ='ZR_tbl_Export_FILE'
DECLARE @Servername varchar(100)='sils-pc\SQLEXPRESS'


SELECT @columnHeader = COALESCE(@columnHeader+',' ,'')+ ''''+column_name +'''' FROM 
guard_security.INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME=@table_name

SELECT @sql = 'bcp "SELECT '+ @columnHeader +' UNION ALL SELECT * FROM ZR_tbl_Export_FILE" queryout 
      c:\ExportExcel\Export_FILE.csv -c -t, -T -S'+ @servername

exec master..xp_cmdshell @sql

pls advice me

Thank  you 

Maideen