Job duties are critical to have on a work reference letter.
Without them, officer will be unable to determine your NOC code, and your eligibility will stumble into deep issues.
You need to provide them one way or another.
See if you can for example:
-Provide your job offer letter which may contain your duties and responsibilities, along with letter of explanation.
-A separate document listing your duties, ideally signed by your supervisor/manager, along with letter of explanation.
-A letter listing duties, and signed by your closest possible colleague, along with letter of explanation.
-In addition to any of these, maybe examples of your work which coincide with your mentioned duties, along with letter of explanation.
-And as last resort, a self attested letter listing your duties, along with a good letter of explanation as to why you were not able to get duties in the original work document
Don't submit your application without your duties listed at least somewhere... CIC are ready to show understanding, but you have to show you did everything in your power to give them the correct information.
Your application needs to be bona fide.
Cases of rejected applications due to missing duties are numerous and common, with officers telling applicants that "they do no meet eligibility criteria as they are not able to verify your NOC".