4.3 is in beta, which means it is being actively developed, it may have bugs, and runtime support may not be complete for every runtime:
http://esotericsoftware.com/spine-versioning#Beta-releases
Likely you will need to use 4.2 until 4.3 is out of beta, or at least is near the end of the beta.
You can export JSON and choose 4.2, then run 4.2 and use Import Data. Let us know if you have trouble doing this.
