In the United States, Canada, and Europe, the Welcome Gift for Platinum Elite and above at Residence Inn is your choice of 500 Marriott Bonvoy points per stay or a F&B item.

Yes, you should have been asked at check-in (unless you checked in online).

If you were not asked at check-in, you could have brought up the subject yourself. Most likely, you would then have been give a choice, probably accompanied with a quick apology, allowing you to decide if you would prefer 500 points (worth about $4) or an item from the pantry, such as a bottle of water (or perhaps a can or beer, if the property allows it).

If the hotel refuses, or gives you hard time, or claims it's the hotel's choice, not yours, then it would make sense to invoke the Guarantee. That would be USD$50. The hotel would deserve to pay the penalty. And the hotel would still need to provide your choice of Platinum Welcome Gift.

But, most likely, the front desk agent just forgot to bring up your Welcome Gift after checking in dozens of other guests that same day -- many of whom lacked sufficient status, some of whom checked in online, and some of whom brought up their preference themselves.