The 0420 code can have several causes.
Since truck is new to you, rec you go underneath and do a visual inspection of the cat and O2 sensors. Look for any obvious damage to the cats or exhaust pipes before the cats. Also check that you have two O2 sensors per cat, they are plugged in, and not using spacers.

 

So i have been getting "failure to communicate" with ECM/PCM when truck would crank turn on then immediately turn off. now couple weeks later did same exact thing only now the code is P0420$07E8

 

So i have been getting "failure to communicate" with ECM/PCM when truck would crank turn on then immediately turn off. now couple weeks later did same exact thing only now the code is P0420$07E8 2015 DD limited 3.6

 

That code is just whining about the cat. Nothing to do with your no start.

If the PCM won't communicate, that is not a good sign.