If you filed 20017, 18 and 19, it's okay if you submit your application and saying no for 2020 taxation. I don't think you need to explain as you need at least 3 years filing and you have them.
let wait till monday. should call the contact center on monday. may be the issue is related to website maintenance.we are having the same issue "failed to submit an application, try again or contact us"
Thank you@redacamden
https://citapply-citdemande.apps.cic.gc.ca/en/register
To register yourself and to apply online