Change all the -s to =. So your code will be:

const RPC = require("discord-rpc")
const client = new RPC.Client({ transport: "ipc" })

const activity = {
  details: "Join me",
  assets: {
    large_image: "images_512x512",
    large__text: "join me",
    small_image: "images_512x512",
    small_text: "just join"
  },
  buttons: [
    {
      "label": "Join",
      "url": "https"
     }
   ],
   timestamps: {start: Date.now()},
   instance: true
};

client.on("ready", () => {
  client.request("SET_ACTIVITY", {pid: process.pid, activity: activity})
  	console.log("DONE");
})

client.login({ clientId: "896042502606450698" });

(I also fixed some small errors)