Ben Pymer Welcome to Microsoft Q & A Community Forum. From the information provided, I assume you are using Foundational CSPM. Defender for Cloud offers foundational multicloud CSPM capabilities for free. These capabilities are automatically enabled by default on any subscription or account that has onboarded to Defender for Cloud. The foundational CSPM includes asset discovery, continuous assessment and security recommendations for posture hardening, compliance with Microsoft Cloud Security Benchmark (MCSB), and a Secure score which measure the current status of your organization's posture.
The optional Defender CSPM plan provides advanced posture management capabilities such as Attack path analysis, Cloud security explorer, advanced threat hunting, security governance capabilities, and also tools to assess your security compliance with a wide range of benchmarks, regulatory standards, and any custom security policies required in your organization, industry, or region.
So, for all governance capabilities, such as "Assign owner" and "Change owner and set ETA", you need to have Defender CSPM plan enabled within Microsoft Defender for cloud. To enable it, you need to go to environmental setting>>Switch on Defender Cloud Security Posture Management (CSPM).
For more information, you can refer below documents.