can't enter expedition mode

What do you mean exactly? Can you provide more detail on the steps you take and how the device responds?

Thank you, JungleJim.

Reset All Settings - Expedition Mode - Auto. Two minutes later, the unit doesn't enter "Expedition Mode."

Doing it this way requires 2 minutes of inactivity before the device enters expedition mode. You cannot push ANY button on the device during this interval. If you do, the two-minute idle timer restarts. The idea is that continuing to interact with the device postpones entry into expedition mode. 

Thank you, twolpert.

I don't press any button, but it still doesn't enter Expedition Mode.

I do it again. This time, the green LED appears. I cannot understand this.

Again, after those actions, the unit doesn't enter "Expedition Mode."

Thank you, Chris.

I didn't express myself clearly.

Expedition Mode > Auto is selected,  pressing the power button on the GPSMAP 66i will give the options of entering expedition mode or powering off, and it will not enter Expedition Mode automatically two minutes later.

If I choose "Prompted," it will act the same.

In short, "Auto" and "Prompted" under "Expedition Mode" act the same.

Software version is 9.90.