how can i delete the missing values from dataset? or i delete from the source befor import it??

There is an operator to Filter Examples and you can specify missing values as a filter option.  You can also use Replace Missing Values if there are only a couple of missings and you have a good method to replace them such as the average or the minimum.

my datadet represent real data so i don't need to replace the missing values, just i need to delete it

Then just use Filter Examples.