The user didn't get the invitation email from Control Center
You invited a new user to your Control Center and the user didn't get the invitation email. When you check Users in Control Center, the user's status is Not signed up.

To resolve the issue, check the user's email spelling:
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Click the user. A window with the user's data opens.
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Check if the Invitation Email is correct.
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Send a new invitation email with the correct email address.