Have been running IoX 5.5.9 on PG3  with Z matter just fine.  I noticed that one wall light, controlled by an Insteon 2476D V 5.4, would no longer communicate with IoX.  Did a factory reset of the switch, but it wouldn't add back to ISY saying it could not determine insteon engine.  So, because it is an older switch, I just purchased and installed a new 2477D switch.  When I try to add it to the ISY, I get the same message.  Any recommendations?  All my other insteon switches work just fine.

The error message you're getting indicates that the PLM is unable to communicate with the switch. What is the 6-digit address of the switch you're trying to add as sometimes the address can be misread.

If the address is correct then I would focus on possible noise on the powerline, 

Have you recently added any new electronics or devices that might be causing interference?

To confirm the noise problem you could connect the 2476D you just removed from the wall to a power cord.

Black>Black

White> White

Ground > Ground

Red - no connection but cover it with a wirenut so it can't touch anything

Plug it in near the PM then try to add it.

I have not added any electronics recently

@SHM

What is the address of the switch you're trying to add?

Is it a new dual band switch that you're trying to add?

Do you have other dual band devices in your system?

Are the 2 legs of your powerline bridged using dual band devices?