This usually means that the client's device need to give Annature permission to access its camera.
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The client needs to check if they've given Annature permission to access their webcam -- If they block permissions to their camera, they can check their browser's settings.

If they do not have a camera or are running into issues with their webcam, they can use the camera on their mobile phone by selecting “Other Options” in the upper right corner and choose “switch to another device”.
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They will be prompted to their mobile phone number and they'll receive a link in their phone to start the process.