Fields of Mistria is a single-player game. The Steam store lists Single-player as a supported feature, and NPC Studio's FAQ does not list online or local co-op.
Community multiplayer mod
The Multiplayer mod is a community project, not an official game mode. Its installation instructions use MOMI, a relay, a mods folder, and the Multiplayer tab in the game's settings. Follow the mod author's installation steps in full and use a separate test save for modded sessions.
Save setup
Use the same mod version and dependencies for every player. Start with a fresh test save, keep a backup of your main save, and keep each player's story progress aligned before joining a session.
Video walkthrough
How to play Fields of Mistria Multiplayer by Viggan Gamer shows the community mod setup.