I’m afraid Republic does not currently support Apple’s iPhone. Still, it’s not clear (to me) what you mean by “why i can’t post to a iphone”? Perhaps more context would help Community to better help you?
If you are having issues with text messages with iPhone users, the following may be applicable:
**Previous iPhone user
**iMessage will hijack messages sent to your number. If you don’t remove your old phone and number from the iMessage database, messages may not be delivered to you. Turn off iMessage has more info on how to disconnect iMessage on your iPhone or online.