I put my headphones in my pc, but I can't use the voice chat, can only hear
First, you check if your headphone micro is working. Then go to Settings> Sound> Voice Chat and turn it off and back on (if it's off, just turn it on). If that does not work, restart Fortnite. If all else fails, you can only contact Epic Games Support.
Maybe the voice chat is on
Go to the language settings and see if you have "Push to Talk" if yes, make it out.