You can show your spine.log for why the JSON can't be exported. Please include the entire log, not just the error portion. The forum will automatically remove your name and email address.
Note you'll need to import the JSON into the same version of Spine that exported it.
If you are doing this a lot, you can do it from the CLI:
Command line interface - Spine User Guide: Export
The input file can be your JSON file instead of a Spine project. For example:
Spine -i your.json -o exported.skel -e export.binary.settings.json
If you are using 4.0 you can specify binary
to use default settings for the binary export, so you don't need a binary export settings JSON file:
Spine -i your.json -o exported.skel -e binary
However, you probably want to import the JSON data into a Spine project and keep the Spine project file. JSON and binary data are not intended for long term storage. See here:
Versioning - Spine User Guide: File storage