You create or edit a protection plan for a Windows machine and enable Cyber Protection features (such as Active Protection, Vulnerability Assessment sections etc.). The plan cannot be saved after making these changes. You don't see any error in the web interface.
If you check HAR logs, you can see 404 error for requests like
Cyber Protection features are carried out by Agent for Windows. If it is not installed on the machine (for example, you only have Agent for Hyper-V installed), a plan with enabled Cyber Protection options cannot be saved.
Edit the plan and disable (uncheck) Cyber Protection features, then you can save the plan.