In your message you said the error message is "Error : Material Number should be filled." and also you confirmed the material number is fille in the Idocs but there is another statement you (just one more this in reciever system no external # is allowed) I don;t understand what is this..
More over t seems you are using extenstion type, can you please check in which user exit you have written the logic. probably you are not filling all required eport parameters in the user exit.
Please check and correct the user-exit where you hare writen the logic and let me know the exit name and logic you have written there..