I have a spine from an game. I have the .atlas.txt, png and json file. But it wont generate the skeletondata.asset file for me in unity. I would love if someone wants to help me.

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You may have already read this, but could you please review the steps listed in the "Importing Spine Assets into Unity" section of the spine-unity runtime documentation?
spine-unity Runtime Documentation: Importing Spine Assets into Unity

If the SkeletonDataAsset is not created even after following the steps, the spine-unity runtime may not have been installed correctly to begin with. Could you please check the documentation and try installing it again?