It's pretty much an innocuous error and easy to fix, the power button that they are referring to is the audio system power button, the one in the upper left hand corner of the infotainment screen. Press and hold that for a few seconds and the infotainment screen will reboot and the error should go away. The error message is poorly worded, don't worry you aren't the first to think it meant the main power button, in fact I think I did also the first time I got the error.
The error has to do with the audio system anti-theft protection, if the head unit loses power then it thinks maybe it was stolen and installed in another car, so it wants to resync and verify that it is in the right car, which requires a reboot of the audio system. Instead of rebooting automatically, which might come at a bad time (like when you are backing out of the driveway) they want the driver to manually initiate the reboot. Which makes sense, but they should improve on the message and make it more clear what button you are supposed to press.
As a side note, if you ever have a bizarre problem with any part of the infotainment system, rebooting by pressing and holding the audio power button will often fix it.