I assume you are using CC 2015 , If you need to export an FLV file, then use the previous version of the application.
See this for an explanation for the change regarding FLV and F4V and for the recommended workflow:
try lowering the previous version , but is not compactible with Yosemite , I have that option ?
As I can export in H264 with RGB + Alpha.
Yes, After Effects CS6 and later are all compatible with Mac OS X v10.10 (Yosemite).