Have obb 1st class tickets, but cannot reserve seats?

Ljubljana is in Slovenia, so you are starting your travel on a Slovenian train, so it is indeed possible that OBB cannot reserve you seats for that. It is also possible that they just haven't loaded the seat inventory yet. Reservations are indeed normally included for 1st class in Austria, but you are not taking a train in Austria...

I would not worry to much. 1st class is usually quieter. And you can always just go to the station (in Ljubljana, or elsewhere) and get a reservation there.

Note that if you were to end up without a seat in 1st class that you will be due a compensation.

Does that mean we won't have guaranteed seats and we may be standing
in the 1st class car?

I rather don't think that the train is sold out but that seat reservations do not work for the Slovenian leg of the journey.
To be sure you might try to get seat reservations from Villach to Salzburg on the same train. That should be possible.

I suspect they are on the direct train EC212, with no changes. That train starts in Zagreb and goes through Croatia, Slovenia, Austria and Germany before ending at Frankfurt.

https://www.putovnica.net/en/transport/train-zagreb-ljubljana-villach-salzburg-munchen-stuttgart-frankfurt-croatian-railways-hz-slovenian-railways-sz-austrian-federal-railways-obb-deutsche-bahn-db

Paul is correct that they can get seat reservations from ÖBB on that train from Villach to Salzburg—the segment in Austria. I just checked, using the “Seat reservation only” button. €3 each.

Thank you all for the replies. Very helpful. You are right!
I just checked their website again and it is now showing the option to purchase seat reservation.

Thank you!