I have a 2013 and had the very same problem from 7/3/2020. Let it sit over the weekend and the flashing D light and check engine light went away. Made an appoinment at the dealer and from what I read online and what was said it is the "4th gear clutch pressure sensor ". Sure enough the Dealer said the same thing. $250 bucks and half day there...they knew right away.
Drove it yesterday all day, about 430 miles....not once has it showed up again. Drove it so far 50 miles today (7/8/2020) no light.
On the first gen Pilot the flashing D is normally resolved with replacing the third and/or fourth gear pressure switch. The code reader tells you which one(s) failed. If you DIY personally I would just replace both since it takes longer to get the front end jacked up compared to the time it takes to replace the switches. Get OEM switches; some have used aftermarket and ended up having to do the job twice.
You might also consider doing a 1x or 3x tranny fluid drain and fill (depending on number of miles since last fluid change). I would do this prior to changing the pressure switch(es).
since the check engine light wouldn’t clear with the code reader, I just disconnected the negative terminals on the battery and code went away driven 45mins so far gonna see if the code come back on. Also I am going to change the transmission fluid. Drain and fill method 3 times. Idk when or if my dad ever did it