I think there is some version mismatch going on here. The Sync2 module requires the following module versions to work on 2.7.0, i guess since i tried adding the sync2 module on refapp 2.13.0 running on the snapshot of platform 2.7.0 and it failed to start requiring the modules below and those versions ;-
- fhir
1.18.0 - atomfeed
1.0.12
Back to the version of sync2 you attempted to run, it requires the properties or module versions as shown here.
I think we would to make sure the sync2 module can run on OpenMRS-2.7.0.(not sure how possible that is since the fhir module was archived 
The other way i guess would be to lower the OpenMRS version you’re trying to run it on since by the time development on that module started, the latest version OpenMRS 2.0.x - (Sync 2.0 for OpenMRS 2.2 - #5 by alalo)
Also, you can take a look at these posts;-