Just encontered this issue after installing the react-native-vector-icons package and run the app on an iOS simulator.

If you're wondering what is happening even though that the library is correctly linked with the iOS project, here is the solution:

Check the info.plist

In my case the problem was that the fonts haven't been added to the info.plist file!

If the UIAppFonts's array is empty, just add the fonts:

<key>UIAppFonts</key>
  <array>
    <string>AntDesign.ttf</string>
    <string>Entypo.ttf</string>
    <string>EvilIcons.ttf</string>
    <string>Feather.ttf</string>
    <string>FontAwesome.ttf</string>
    <string>FontAwesome5_Brands.ttf</string>
    <string>FontAwesome5_Regular.ttf</string>
    <string>FontAwesome5_Solid.ttf</string>
    <string>Foundation.ttf</string>
    <string>Ionicons.ttf</string>
    <string>MaterialIcons.ttf</string>
    <string>MaterialCommunityIcons.ttf</string>
    <string>SimpleLineIcons.ttf</string>
    <string>Octicons.ttf</string>
    <string>Zocial.ttf</string>
  </array>

Happy coding!