I have everything in tech and all of nature but how do get the magic it says faction has been chosen

If you want to change of Faction you must start another tribe!
There is one faction, one "manner to play", by tribe!

Laurence is correct. When you start a tribe you accumulate enough points to purchase tech levels. Now you have the choice to either take the path of "Nature" or "Magic". You cannot do both in any one tribe. In fact, when you purchase a level of Nuture or Magic, it will ask you if you are sure.

So the only way to play the other "faction" (Magic or Nuture), you would need to start a new tribe and and purchase the other path.