Hi @Kumar Balaji, Madan ,
May I ask you some questions first to confirm something?
Does the error appear when logging in to the SharePoint site?
Is there an error code similar to the one marked in the picture below, can you provide it?
What solutions have you tried, can you briefly describe it?
It is recommended that you go to the Active users and Guest users of the Microsoft 365 admin center as an admin to confirm whether the account exists. If it doesn't exist, please add it. If it already exists, please delete it and add it again. See if the situation is resolved after the operation. For related operations, please refer to here.
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