Unable to confirm I am an adult by uploading ID/ passport.

Which country? You may have other ways.

Region-specific rules for managing an Apple Account - Apple Support

Community threads outlining age verification in the UK and ID recognition troubleshooting tips:

iPhone not accepting ID for age verificat… - Apple Community

Can't confirm age in UK - Apple Community

"Unable to confirm I am an adult by uploading ID/ passport.: I am unable to confirm I am an adult. I continually get a message saying unable to upload image of ID/ passport."

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Await the Time Allotted:

There are no reported issues on Apple's side, as of this reply. See here: Apple - Support - System Status. So, being prompted, you'll have to wait out the time allotted. Important: Let it be, or you may be locked out from doing so for a greater period of time, the more you try this unsuccessfully.

Use the Device this Photo was Created with:

Did you take this using an iPhone? If so, perform this using the iPhone, and nothing but.

Brave Search Engine: cant add a valid proof to get a passport photo on mac site:discussions.apple.com