To generate a Spine file from a 3.6.53 JSON file and an image atlas folder, and then upgrade it to version 3.8.99, you need to use the correct command line options to specify the import action. The error you encountered indicates that the action was not specified. Here's how you can proceed:
Import the JSON to create a Spine project file: Use the --import or -r option to specify the import action. This will create a .spine project file from your JSON file.
Spine -u 3.6.53 -i /path/to/skeleton-3.6.json -o /path/to/skeleton.spine -r
Upgrade the Spine project to version 3.8.99: Once you have the .spine file, you can open it with Spine 3.8.99 and save it, or use the CLI to export it to JSON.
Export the upgraded project to JSON: Use the following command to export the project to a JSON file using Spine 3.8.99.
Spine -u 3.8.99 -i /path/to/skeleton.spine -o /path/to/skeleton-3.8.json -e json
Make sure that the paths to your files are correct and that you have the necessary permissions to read and write to those locations.
