فَأْتُوا بِسُورَةٍ مِنْ مِثْلِهِ
Then produce a sūrah like it.
The Qurʾānic challenge “produce a sūrah like it” serves taʿjīz: to demonstrate that the addressees cannot meet the challenge. It is not merely an ordinary instruction to undertake a task.
In a normal request, an achievable act is expected. Here the discourse of challenge and opposition turns the imperative into a demonstration of inability, and the outcome is evidence of the exceptional status of the Qurʾān.