Hi uss_atlantis,
and welcome to this Apple (user-to-user) Community!
Am no expert in this area, yet your question to the community deserves some response.
Fortunately, Apple support's online articles may help shed light on the situation
Re: " How do I add my MacBook Pro to AppleCare One?"
" When i tried adding my 11 months old MacBook Pro with AppleCare+, it wouldn't let me."
The website: AppleCare - Apple
says:
"Add devices you already own. Most devices less than four years old and in good working condition can be added to your plan — right from your iPhone, iPad, or Mac. "
"(3)To verify “good condition,” products may be required to undergo a diagnostic check using a customer’s iPhone or iPad, or at an Apple Store, prior to being added to the plan."
Apple's Announcement: Apple introduces AppleCare One, streamlining coverage into a single plan - Apple
says:
"Only devices in the customer’s Apple Account can be covered under AppleCare One"
You may want to double-check you are signed in to your Mac with your correct Apple Account details
If needed: See: Sign in to your Apple Account on the web or in your device settings - Apple Support
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Apple Support's online article: Add or remove a device from your AppleCare One plan - Apple Support
Eg says:
"Add a device to your AppleCare One plan
- On your device, go to Settings > General > AppleCare & Warranty.
- In the AppleCare One Devices section, select Add Device.
- Next, choose the device you want to add.
- If the device appears under Ready to Add, follow the onscreen prompts to add it to your plan.*
- If the device appears under Action Needed, you might need to complete a device check before it can be added to your plan.
- If you don't see one of your devices, tap or click "Don't see one of your devices?" to learn more. If you still don't see your device in the "Not Eligible for Plan" section, you might need to sign in to your Apple Account on that device or pair it with one of your other Apple devices associated with your account.
*If you add a device that has AppleCare+ or AppleCare+ with Theft and Loss coverage, it will be covered by AppleCare One and your previous AppleCare+ plan will be canceled."
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Re: "When I tried adding directly from my MacBook Pro,
it instructed me to do it from my iPhone, as my MacBook Pro requires a check".
Seems Apple may need to check on the MacBook Pro's condition before it can be added, (eg: perhaps remotely, or in person at an Apple Store.) Eg: Could be damaged already, linked to a different Apple account, or covered under a business account.
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Re: " I feel like the person I talked to at Apple Support was clueless."
To contact AppleCare's experts: You can Call Apple Support at 1-800-APLCARE (or use the app)
AppleCare Products - Apple
AppleCare - Apple
.
All the best :-)