Only constant grids are selected for rendering a given volume grid. Unable to determine size of the grid, please select at least one grid from a file.

This error message appears if all enabled grid sources are set to "constant" (Emission, Scattering, Absorption). With such configuration, Corona Renderer is not able to render the object correctly, and this results in the object not being rendered at all. 

Solution: make sure that at least one of the enabled grid sources is set to other mode than "constant". The object will be then rendered correctly, based on the currently selected settings. 

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