You don't activate an SD card. You just insert it and it gets mounted. The only cards that get 'activated', are SIM cards, so you can get service.
Now, if you are trying to set up the SD card to accept apps, etc., which is a common question, then you would need 2 things (plus the instructions I am including here). 1. The phone has to be running, or upgradable to, Android "Marshmallow." 2. The manufacturer must not have restricted the option to make this change on that device. (Which does happen.) Follow the instructions below. If they don't work for you, you may have a device that has been restricted by the manufacturer.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank...