I had the exact same problem, and I shared my experience in another thread.

Their rewards programs are combined, but the booking systems are still separate. When you try to book a SPG property, you are redirected to the old SPG site, which only offers the option of points, not certificates.

When I called them (called many departments actually, none was able to help), they said this functionality is not yet available. Worst part is, no ETA. They cannot even say whether they are working on it or plan to work on it. As far as I know, they could be satisfied with their current workaround of redirecting to SPG site for SPG bookings and won't actually integrate the two booking systems for months or years.

I offered them the idea of booking the hotel using points, and then have them convert the certificate into the same number of points, thus crediting the points back to me and effectively using the certificate. They declined saying converting certificate into points is not possible, which is clearly not true since people have shared experience where they did manage to convert certificates into points in exceptional cases.

I'm going to give them a few more days. If this functionality is still not available, I'll be more forceful in my demands for implementing my method since they openly communicated before that Marriott certificates will be available for SPG properties going forward and they are reneging on their promise.