Improve expand command error message for non-nested fields

[penghuo]

Description

Improved the error message for the expand command to clearly indicate that only nested array fields are supported.

Related Issues

Resolves #5065

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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