That message usually means Writing Tools doesn’t have enough selected text to work with.
In Mail, try selecting a complete sentence or, better yet, a full paragraph. Then Control-click the selection and choose Writing Tools > Show Writing Tools. Apple’s instructions also specify selecting the text you want to rewrite, proofread, or summarize. → Use Writing Tools with Apple Intelligence on Mac
If you’re already selecting a full paragraph and still get the same message, try that same text in Notes. If it works there, I’d quit and reopen Mail and test again. If it fails in Notes too, check System Settings > Apple Intelligence & Siri and make sure Apple Intelligence still shows as enabled after the software reload.
→ Use Writing Tools to write, improve, and summarize content on Mac - Apple Support