As a sales manager, my job is to suggest a product and make an agreement with customer. = good sentence

Now, I’m joining the military project this year.

> I joined a military project this year. = now and this year are time expressions which express present time use only one

My company is developing a program about a missile defense system. = good sentence

In this project, I am responsible as a sales manager. = good sentence

I always attend a meeting of main customers and review the contact and cost.

> I always attend meetings with main customers and review the contact and cost.

Sometimes, I decide a major decision in the field.

> Sometimes I make a major decision in the field. 

The relationship with customers is not bad. = good sentence

The project finished operational test and evaluation last week. = good sentence

Finally, the determination remains about the test results next month.

> But I have to keep my determination for the test result next month.

Thank you for sending me your composition Mr.Lee. I realized that in the class I misunderstood some information. 
Most of the sentences you made were correct. Let's work on improving your speech for better clarity and accuracy.