@HHmayo 

I suggest a number of things:

Become more familiar with the correct terms and what your wires are.  Examples:

1. There is no such thing as a "line/load" wire.  Kasa does you no favors in their labeling.   It's either a line side or a load side.  Not both.  Ever.
2. A neutral, while intended for a different function, goes to ground just as a ground does.  There is no way one is connected to a line or a load.

Do not focus on wire color.  EG:  While often white is neutral and black is line, this is FAR from universal.   This is particularly true of 3-way and 4-way switches.   We can say that usually when a bundle is connected it tends to be correct vis a vis the "typical" color meaning.   Meaning most likely your white bundle in back is neutral.

Buy a voltmeter and learn how to use it.

Do all those things before you hurt yourself or start burning stuff up.

The odds, although you need to check all this with a voltmeter, are that your two blacks are line and load.  Drop a V-meter on and see which one is hot when the circuit is live.  That's "line".  Connect it to line-in on the Kasa.  The other one is "load".  Connect it to Kasa.

Stub in a new white wire to the correct neutral bundle.  Check w/meter, but likely that white bundle in back has no voltage when checked to ground, and when ohm's checked to ground should be basically zero.  If so, yea, that's neutral.

Connect the ground.   Test.

I suggest you not attempt any 3 way switches.