In order to synchronise, please upgrade your application to version 3.7.0+
Except the latest release is 3.6.15.
What is this insanity? Why are you breaking my note syncing until some random time in the future you will release a new version? This is shit man, why are you doing this? I have all my notes in this app..
This is true. The latest release version is 3.6.15. However at some point you have installed a pre-release client and it has updated the sync system and so all clients now have to be v3.7.x.
Pre-releases can be found here.
Joplin v3.7 is slated to be published as a release version around the first week of September.