Well it wasn't Apple Pay.

It was a Google Flight fare that didn't match what was in the DL system.

In fact if I searched manually on Delta, the DL fare was over $400 more.

So the DL system didn't want to process it.

I went to AF site and got the lower fare, though on Google Flights, the booking options showed DL booking option was actually about $30 less than the AF booking option, which itself was over $400 less than searching manually on Delta.com.