Hello R S,
Thank you for contacting Microsoft Q&A Community
Based on the error, the user account doesn't exist in the tenant where you are trying to access.
If you have been able to access the tenant where the subscription is before now, you want to check if you are authenticated into the right tenant.
To do this, try switching your tenant and see if you can see another tenant that has been linked with this user account. Click on your user profile then click on switch directory
Alternatively, try forcing the authentication if you know the tenant where the subscription resides by accessing this URL: portal.azure.com/yourtenantid
Let me know if this helps.
Regards