It will not connect

Five checks, in the order that finds the cause fastest.

1. Is the far computer online?

Open Devices. If it is not green there, nothing you do on this side will help.

On, but showing offline? Usually:

Sharing was never turned on. It is a switch, and installing does not throw it. See sharing.
The machine is asleep.
It is signed in to a different account.
On Linux, the service is running outside a graphical session.

2. Same account on both ends?

Check the email in both apps. You only see computers on your own account.

3. Restart both apps

Genuinely worth doing early. If the far computer has been on for weeks, restart it too.

4. Close the version gap

Compare the version in each app, and what Devices says each one last reported. A large gap is worth closing first. See updates.

5. Look at the network in front of you

Office or campus network? A firewall blocking outgoing UDP forces every session onto a relay, and some block that too. Ask for outgoing UDP to be allowed. Nothing incoming is needed.
Mobile data? It usually works, through a relay. See direct and relay.
Guest Wi-Fi with a sign-in page? Finish the sign-in first.

Linux checks

systemctl --user status realframe-host    # is it running
journalctl --user -u realframe-host -f    # what does it say
echo "$XDG_SESSION_TYPE"                  # want: x11

A host with no graphical session has no screen to send, and Wayland sessions cannot be captured yet.

Still nothing

Email hello@realframe.io with the four facts from the index page.