If you have an adapter is the card recognised in a PC? If yes it could be that the slot in the phone is faulty as it should recognise any MicroSD card up to 200GB in size.
You could try formatting the card in the PC to exFAT as that's the format the phone uses to see if that makes a difference. However, with Marshmallow, it's best if you can get the card recognised to format it in the phone as it will then give it the required access permissions. It will work if formatted in a PC but then appears to constantly ask for permission to write to the card when accessing it for storage.