Sorry, it is not supported to import JSON from newer versions of Spine into older versions. You can try it and it may work (there aren't many JSON changes between 3.7 and 3.6). If it doesn't work, you will need to modify the JSON so that the older version of Spine understands it.
Note that the binary format is more sensitive to changes and it is very, very unlikely that an older Spine version can understand a binary export from a newer version.
3.7 will come out of beta in a few days. Another option is to update the runtime to 3.7.