Hi @Amalia  
Thanks for your question. 

"Code 21"  means the Canada Revenue Agency was unable to accept your return, as your NETFILE Access code did not match CRA’s records. Please review your latest Notice of Assessment and ensure the access code listed on the Notice is being correctly entered.
If you are entering the correct number, or you do not have an access code or copy of your Notice of Assessment please contact CRA at 1-800-959-8281 for assistance.