I am not familiar with said tutorial - but the error denotes that another process (probably the same express app) is already listening on port 3000.
On Linux you can list all running process with the command ps aux. Look for another node process. If there is none - you can find which processes are listening on which ports by running lsof -Pnl +M -i4 for ipv4 addresses and lsof -Pnl +M -i6 for ipv6.
Or simply do curl http://localhost:3000 in the Digitalocean droplet.
