My fitbit Versa is not charging

It could be either, it’s hard to say for certain. Was it working fine previously? If so, the battery seems to be fine unless it was left out for ages, depleting the battery to the point where you have to charge it for a while for anything to happen. It could also just be the charger as if it is not going on charge or recognising that it should be charging then that may be your issue. I would say the charger is more likely and one to start with but if still nothing, then definitely battery.