Final answer:
The TSO command that can be entered at the READY prompt to diagnose an error message is Analyze.
Step-by-step explanation:
The TSO command that can be entered at the READY prompt to diagnose an error message is Analyze. This command is used to analyze and identify errors in the system. By running the Analyze command, the system will provide information about the error, such as the error code, description, and possible causes.