If the Mach trim is inoperative and the autopilot is engaged, what is the speed limitation?

When the Mach trim is inoperative and the autopilot is engaged, there are generally no specific speed limitations imposed. The autopilot system is designed to maintain control of the aircraft under various conditions, including managing speed. The Mach trim system is essential for large variations in speed at higher altitudes where Mach number becomes relevant. With it inoperative, pilots must remain vigilant and manage speed manually to avoid potential aerodynamic issues, especially during speed transitions. However, the absence of explicit limitations from the autopilot in this specific context indicates that pilots can safely operate without additional speed restrictions beyond standard operational practices. This is why the answer reflects no speed limitation in this scenario.