You will have no trouble leaving Canada, just returning to Canada.
As a permanent resident, you are ineligible for an eTA. If you apply, it will be refused.
IRCC recently removed urgency processing for previously booked vacations.
You have two options:
- Apply for a Permanent Resident Travel Document (PRTD) in Cuba (takes 2-3 weeks usually and you must apply in person in Cuba)
- Change your flight to an airport in the US near the Canadian border and drive back across the border into Canada (you do not need a PR Card to enter Canada by car).