I have a single on/off switch controlling a single light. I’ve checked carefully, and have ONLY one 3-wire Romex with black, white, and bare copper ground. (No white neutral wire hidden in box.) Am I correct (as the box says) that I CANNOT install an Insteon 2477S? Is there a different Insteon switch that would work? (Calif. house built in 1988.)
You can't. Neutral is not available in that box.
This is a switch loop, and currently white and black are always-hot and switched-hot. The proper method is to use white as always-hot, to make it more apparent that it is not a neutral (i.e. it's always hot).
Your best bet is to go up into the lamp box and re-task the black and white wires to be always-hot and neutral (white must become the neutral, so make black always-hot). The switch's red wire goes to nothing at all. Then, obtain an Insteon inline module and place that up in the ceiling rose of the lamp. It will connect to always-hot, neutral and switched-hot to the lamp. (the lamp's other wire goes to neutral, of course). Then you pair the module to the switch.
It may also be possible to obtain LED bulbs with the module built into the bulb.