Start With the Official Fixes
If MECCHA CHAMELEON will not connect, do not start with random firewall edits or generic “connection error” videos. The most important official fix is simple: everyone must run the same game version. The v1.1.0 patch notes state that matchmaking is only available between players on the same version and tell players to restart the game to update.
That means a lobby can fail even when both players own the game, both are online, and both know the password. If one player has a pending Steam update and the other does not, the game can block multiplayer by design. v1.2.0 repeats the same idea in practical terms by warning that players must update the game to avoid multiplayer issues.
This page separates fixes into three confidence levels: official patch facts, safe Steam maintenance steps, and community-reported workarounds. Use them in that order.