Hello kusaj,
Are you trying to sync via USB cable or Wi-Fi? Here are some steps you can try to address the issue. The screen shot shows an iPad, but the instructions are the same for your iPhone, which is also an iOS device:
iTunes 10.5 and later: Troubleshooting iTunes Wi-Fi syncing - Apple Support
https://support.apple.com/en-us/HT204089
To sync your content to your iOS device through Wi-Fi, you must turn on the option in iTunes:
- Connect your iOS device to your computer using the Apple USB cable.
- Open iTunes and select your device on the left side of the iTunes window. In iTunes 11, your device will be in the upper-right. You can also find your device by expanding the sidebar from the View menu.
- Click the Summary tab.

- Scroll to the Options section and select "Sync with this [iOS device] over Wi-Fi."

- Click Apply.
Quit and reopen iTunes on your computer. Then, restart your iOS device.
Restart your home network router using the manufacturer's instructions. You may need to disconnect the power cord for 30 seconds or more.
Note: During this time, Internet services, such as a VoIP-based phone, won't work, and you may need to reset additional network devices.
Cheers,