I have a general question. Does check-in online affect the cancelation policy (such as refund amount and etc.) or they are not related together? I need to know this since I need to check-in within 48 hours before flight, but I should do pcr test and I am not sure still I will go or not. Anyway if I check in online can I cancel the ticket later with the same condition? I think this is not related to the specific airline and the answer is same.
This depends very much on the fare rules of your specific ticket and terms and conditions of the specific airline. You can certainly cancel the ticket after check in but what exactly happens depends on the details. If you don't cancel and don't fly you are a "no show". Some airlines have different rules for "no shows" vs "cancel". Some airlines also change rules 24 hours before departure which may or may not be coupled to check in.
I once had to change a return flight and doing this after check-in (for the outbound leg) was substantially cheaper than doing it before check-in. So it can make a difference.
This is general question and the dependency of check in to cancelation
There is no general answer. Cancellation and refund rules are notoriously complicated. I strongly advise reading the fare rules for the ticket you are considering.
Checking in online or offline will prevent the passenger from making any free change to their flight with VivaAerobus. From their terms and conditions:
Said changes may only be made until 4 (four) hours before the time scheduled for the Service, provided your boarding pass has not already been created.
This made me wonder What's the point of checking in several days before a flight? (tldr: early check-in helps avoid missing the flight since VivaAerobus notoriously overbook them. When I missed my flight last Christmas because of overbooking, the airline employee confirmed it.).