Why this appear ?

Did you have the battery replaced for your phone? If do did, a non-genuine Apple battery was installed and you may not be able to see accurate info like Battery Health for your phone.

If you have not yet replaced the battery, then it probably needs to be replaced. Check as follows:

Settings > Battery > Battery Health

If the capacity is less than 80%, the battery needs to be replaced at an Apple Store or authorized Apple Service shop.

jaymazon ~ If you've had the battery replaced, verify the repair provider here:

Repair Provider Verification - Official Apple Support

...found by searching Apple's Knowledge Base.

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