seat assignment on LATAM codeshare ticket with AA metal
 

How do I select a seat assignment on an American Airlines operated flight ticketed by LATAM and with a LATAM codeshare?

You need to find the PNR recorded in the AA reservation system, which will be different than LATAM's PNR. I would expect the information is buried somewhere in the info that you received from LATAM or your travel agent when you booked. You then go onto AA.com and search by your name and PNR.

If you attached an AAdvantage number with your reservation, then you should just be able to login and look under my trips. I was in the same boat on a flight a couple of months ago and that's how I did it.

I use Qantas website to get the AA reference code (although I have never tried it with LATAM, but other airlines have worked well).

Also AA's website may not complete the association on the first try, but it always seem to work on the 2nd try about an hour later.

Good luck.