A possible cause can be if the linked accounts are showing as Unknown because they are missing the IAM roles. The platform requires API access to collect data for Metrics and Performance reports.
The solution is to Set Up Read-Only IAM Role for Linked Accounts according to these steps:
https://techdocs.broadcom.com/us/en/ca-enterprise-software/it-operations-management/cloudhealth/saas/index/getting-started-with-tanzu-cloudhealth-getting-started-index/getting-started-with-tanzu-cloudhealth-aws-quick-start.html#configuring-linked-aws-account
If the linked accounts are showing as Unknown at channel customer that has a FullFamily billing block, then use the steps in this knowledge base article: https://knowledge.broadcom.com/external/article?articleNumber=284065