Rviz2 Frame Problem

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Hi; I am using ROS2-Foxy distribution. I want to visualize the data of my IMU on Rviz. When I run the Rviz2, in the default screen, the fixed frame is "MAP" and I had global status warn. To change the fixed frame, I tried to click down-list button near the text of "MAP". But, this button did not work. I didn't have any options other than "MAP" for this fixed frame. How can I add another fixed frame other than "MAP".