STRIX internally needs a list of GameObjects that have StrixReplicator attached and are subject for replication. Strix Unity SDK scans assets in the project and creates the list automatically when you switch the Unity Editor to Play Mode.
It may take some long time if you have a lot of assets. You can check off Automatic Prefab Scanning on the Strix pull-down menu to disable the automatic creation of the list so that you can enter the Play Mode faster. Please note, however, that you need to update the list manually by choosing Scan Prefabs Manually on the same pull-down menu after adding a GameObject subject for replication (with StrixReplicator attached). If you forget the manual update, a correct multiplayer may be damaged.
The Strix pull-down menu is available in Strix Unity SDK version 1.5.1 or later.