The problem is that your atlas is missing an atlas region
Did you perhaps accidentally delete an attachment image before exporting your skeleton assets?
I also just noticed that the
Resolve atlases.. button did not load recently created (currently imported) atlas assets that are selected in the file browser on any newer Unity versions. Thus the error message was not helpful at all, listing all atlas regions as missing as if no atlas asset was selected at all. We are sorry for the inconvenience! We just fixed this issue and improved the error description, it now shows the missing regions again as displayed in the above screenshot.
New 4.0 and 4.1-beta unitypackages are available for download here as usual:
Spine Unity Download
This issue has been tracked under this issue ticket URL: