Having trouble generating 3D model

Thank you all for this great software and great forum
I am having trouble generating STL model from CT scan.
Once I upload the CT scan, I go to volume rendering and choose Bone as a preset tool. However, it worked only once with me after that I can’t do it again. I can not have a 3D model to crop from a 3D model

Could anyone please help me with this?

Thanks

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