What to do when an eTA is denied

If your eTA is denied there could be a number of different reasons for this. One reason could be due to the answers provided in the eligibility questions you submitted in your application, including criminal convictions and medical details. Unfortunately the reason for the refusal will not be stated in your email notification.
If you took a copy of your answers then you can check through your application to see if you made any mistakes but you should never reapply with false information in an attempt to get your eTA approved. You may be able to apply for a visa but if your application has been denied due to security reasons or health reasons then it may not be possible to get travel authorisation and will not be able to visit Canada.
If you think that your application was refused because you made a mistake on the application then you can submit a new application 10 days after you receive your notification email but you will need to pay the fee again.