Hello 未来 坂野.
As per the description, we understand that you are getting below error while accessing Microsoft Defender portal.
"The selected user account does not exist in tenant 'MicrosoftServices' and cannot access the application in that tenant."
Please let us know if this issue is only with single user or all users.
And also let us know if effected user is able to access Entra ID portal. https://portal.azure.com
If he is able to access Entra ID portal and issue is only with Microsoft Defender Portal, I suggest you to check if the trial has expired.
After 30 days, charges begin in accordance with the plans enabled in your environment. To learn more about these plans and their costs, see the Defender for Cloud pricing page
https://azure.microsoft.com/en-us/pricing/details/defender-for-cloud/
Please refer the below document that shows minimum requirements for Microsoft Defender for Cloud.
https://learn.microsoft.com/en-us/defender-endpoint/minimum-requirements