when log in to Cisco Packet Tracer, NetAcad login required, got error

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Every few days when I log into the Cisco Packet Tracer simulator, I need to log into NetAcad, but after logging into NetAcad, I get a prompt like this. What should I do?

This sign-in page is always non-existent and the following Cisco Packet Tracer simulator is not available

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Do you know why?