Hi,
If your customer experiences a technical issue on the Volume License Service Center when trying to Sign In, register an Agreement, locate a volume license key or activate a service, the VLSC FAQ is a great resource to help your customer resolve the issue himsel/herself.
If your customer needs assistance, I suggest to contact the support here:
https://learn.microsoft.com/en-us/licensing/contact-us
More information can also be found here:
https://learn.microsoft.com/en-us/licensing/product-technical-support
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Best regards,
Leon