This issue has been happening to many volt owners as well as many other chevy vehicle models.
The cause is not 100% pinned down yet but I taken some actions which lead me to strongly believe the cause of "shift to Park" messages especially when they start to occur intermittently, are because of a small microswitch built into the shifter which is supposed to sense when the car is in Park. It starts becoming intermittent and then finally fails OPEN once and for all. Chevy appears to have decided NOT to cover the fix under warranty They completely replace the shifter, which obviously then has a new switch in it. Some people have paid upwards of $450 to get this fixed, all because of a $5 microswitch.
Some with this problem as intermittent have found that squeezing the shifter handle (volt shifter handles have a button built into the knob at the top), if they squeeze this "button" a few times they can often resolve the problem, temporarily, until the next time. This is because I believe (from having opened things up and looked at the shifter and this small switch) squeezing the shifter know ALSO moves the switch through it's on and off positions. Since the switch has become intermittent, it is not surprising to me that tickling it a little, occasionally causes the switch to work again, at least once in a while. It all depends on how bad the switch contacts are degraded.
Having been through several troubleshooting steps on my own and having even removed and reinstalled this little switch, I have decided to buy several new switches so that I can repair my vehicle when my switch goes bad. Yes, my switch is not actually bad yet, I just decided to investigate the problem BEFORE it left me or my wife stranded somewhere.
Like I say, I cannot say with 100% certainty TO YOU that this microswitch is the cause, but I do say that to myself as I have troubleshot this issue electrically and mechanically within the last 7 days.
Hope this helps and doesn't confuse.....