Hey there anicole68,
Thanks for reaching out to the Apple Support Communities. If you've confirmed that Sync Library is enabled on both your Mac and iPhone, try restarting your iPhone with the steps shown in this link here: Restart your iPhone - Apple Support
This can help with refreshing the connection with the Apple services on your iPhone, which may be able to help with resolving the issue.
You'll also want to ensure that your iPhone and Mac are both connected to the Internet over Wi-Fi, cellular, or using Ethernet (Mac only).
If still having issues after the steps above, check out this additional resource that can provide some more steps to help with troubleshooting the issue: If songs are missing from your music library after you turn on Sync Library - Apple Support
Keep us updated on the results.
Take care!