Two common errors you might encounter are:
Create match failed: Failed to access NetworkDB appId=0
This occurs if you have not linked the project with Unity Services. To link your project with Unity Services, see Setting up your project for Unity Services.
Create match failed: Failed because of invalid projectid setup. Please verify project setup at https://multiplayer.unity3d.com
This occurs if you have not enabled the Multiplayer Service. To fix this, make sure you have set up your project for Multiplayer Services correctly.