--Get rid of the brackets :<-->: [IndexName] --> IndexName
set @IndexNameExists = substring(@IndexName, 2, len(@IndexName) - 2)
set @SQLCmdExist = N'select @IndexExists = name from ' + @DBName + '.sys.indexes where name = ''' + @IndexNameExists + ''''
set @PDExist = N'@IndexExists varchar(128) output'
exec sp_executesql @SQLCmdExist, @PDExist, @IndexExists output

set @SQLCmdExist = N'select @IndexExists = name from ' + QUOTENAME(@DBName) + '.sys.indexes where name = ''' + @IndexNameExists + ''''