How to get Resin?

You can find Resin in Sandcastle, you'll need the iron axe which you get by upgrading the logging function of a normal axe, there are more details about this here: https://www.appgamer.com/lifeafter/answers/9548-how-to-make-an-iron-axe

Once you have the iron axe you'll need to chop down trees with it to get Resin.