[Vuejs]-Why do i keep getting "connection to the server was unsuccessful. (http://localhost:12896/)"?

https://guide.meteor.com/mobile.html#running-your-app:

meteor run will try to detect the local IP address of the computer
running the command automatically. If this fails, or if you would like
your mobile app to connect to a different server, you can specify an
address using the –mobile-server option.

So yes, I would try without the --server option first. If that fails, you will need to find an IP address or hostname that the emulator can resolve. Presumably your local IP address, check ifconfig, would work for this. You can also use

ip route get 8.8.8.8 | grep via | sed 's/.*via \(\S*\).*/\1/'

to get the IP address of the device you currently use to connect to the internet, which should work.