Why I can't en passant ?

It just worked in Analysis, so I'd say it's a problem on your end.

He's doing it on his phone.

It should work via drag and drop.

Could just be a problem with the app (wouldn't be surprised, it's super glitchy)

Sometimes clearing browser cache, quitting browser or even rebooting PC may help. Now, you know how to test: use analysis mode.

So you're doing this on your phone, right?

So you are trolling all of us right?

this one example I've found yesterday.

https://www.chess.com/live/game/2144542240

The game suddenly paused after in en-passant condition but time keep running. That's what I experienced often..and often..

I assume it's some of bugs

How do you get that from the game? The black player was going to lose a bishop and probably let time expire. Sure, it's a bishop for 2 pawns or a passed pawn that wouldn't last long but still. And black was already down 2 pawns. Doesn't mean the player couldn't en passant, just that they didn't.

 have you tried changing your browser?

I'm pretty confidant it timed out because the player decided either not to resign and let time run or was looking for a way to salvage the position and just ran out of time. It didn't have anything to do with en passant. 

It's 1 minute master's game bro, like I always play. There were many audiences watched the game. They will be fair play to resign. Even when they let the time run (no move), the system auto resign will be applied because it still has much time to play. Or maybe, when they leave the game, it will be end automatically. But the audiences, including me still there waiting until time's running out. And I just realized it's en passant error like I often experience. Black was lose because can't move any pieces caused by system error. I suggest to anyone try to play 1 minute with smart phone, I hope you all will found this dilemma.

This is the recommended procedure for moving pieces.  It greatly reduces "Mouse Slips"

 Just so I understand what happened....

You were playing the game on your phone.

You were the black pieces.

Your opponent moved b2-b4.

After that move, the game freezes.

You think that the reason for the freeze is because an en passant move is possible.

Correct?

All of this makes me wonder if he is touching the right squares.  I've seen beginners not truly understand en passant.

If a White pawn is on d5, and Black moves e7-e5, and he says he's doing touch start square - touch destination square, which is he doing?

Touching d5 - e5

Touching d5 - e6

If he's doing the former, there's his problem!

Have you updated to the newest Android app? Also, clear your app cache and if it still happens, clear the app data and start again.

I've never had an issue doing en passant on the Android app and never had it freeze when the possibility appears. 

edit ... I suggested that before but don't know if you tried it.

It doesn't prove anything. Millions of games are played on the site every day and the number of games that have one or more en passant postions are likely in the tens of thousands, if not more.

While I can't discount that you are having an issue, it isn't one that is impacting the vast majority of members or games.