I need to know how I can add one more car to my apple wallet

Confirm your region is set correctly.

Change your Apple ID country or region - Apple SupportAppleOfficial Apple Support 

If you’ve confirmed the above, please update your iPhone to the latest iOS, 16.5.1 or higher. Older iPhones may only support 15.7.5, but Apple Pay will function.

Update iOS on iPhone - Apple Support

Once updated, please do a forced restart of your iPhone.

Force restart iPhone - Apple Support

Make sure your are signed into your Apple ID/iCloud account on your iPhone.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

Follow the steps in this support article for adding credit/debit cards to your Wallet.

Set up Apple Pay - Apple Support

If you experience any issues along the way, please note the error message and at what stage you experienced the error. Please reply anytime if something is confusing.