Greetings, Chinchin999.
After reviewing your post, it appears that you accidentally filed a claim against your iPhone instead of arranging an appointment. Now your iPhone is marked as lost and you're unable to use it at all. You'd like to cancel the claim that you've reported, and we're glad to help with this.
File a claim if you have AppleCare+ with Theft and Loss coverage -- This article provides the information and steps that you need under "How to check the status of your claim". You'll need to contact Assurant to cancel the claim. Use the phone number or the website. Also:
"If you cancel your claim
If you cancel your claim, your iPhone will be usable again, but you must set it up like a new device, including a new passcode, Touch ID, or Face ID. To get the data previously stored on your iPhone, restore your iPhone from a backup.
You can't cancel the claim if a replacement device has shipped."
Take care.