If you have fixed elements in your Sketch prototype, you'll need to do the following:
If you've already imported your Sketch prototype to Maze: Please go back to Sketch and disable the fixed elements in your prototype. After that, save the changes to Sketch Cloud, and refresh your prototype in Maze.
If you haven't imported your prototype to Maze yet: Please disable the fixed elements in Sketch and save the changes to Sketch Cloud. After that, you can import a Sketch prototype and continue creating your maze.