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.