+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
But it’s only saying that you can submit application through mail but no where mentioning that you can submit form online also

You are right. I thought I saw a note that you could. I guess I was wrong. Renewing your PR card can only be done by mail/paper application. This is one of the reasons to prevent people from applying for PR card online while residing outside Canada since the requirement is that you are in Canada when you apply.
 
I am having an issue, I am sure I have completed all the required documents but still the submit button not active to submit, any idea what is wrong
 
I was in same boat, finally applied paper based application and my renewd card was sent to me in 25 days (after the date of receipt of my application).
 
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