That is nonsense. They are required to sell you a reservation when you ask for it, regardless of the pass you have. At which station was this and around which time? Sounds like someone who hasn't been informed correctly or invented/supposed this rule themselves.

Go back to a ticket office, I'd say. When they say the same thing again:

• Explain that Interrail explicitly say, also in the app, that you can book at the station. Also in Romania. In the app, go to More > Seat reservations, enter Romania, make sure "I have a mobile pass” is selected and tap on Search. The first option shown is the Interrail website; the second one is "book at major train stations”.
• Explain that it is impossible to book online since delivery time is more than a month. Also, many trains just cannot be booked online.
• Ask them to show where this supposed rule is written.
• Ask to speak with the manager (I would only ask that if they continue to insist.)

Bonjour,

Alors aujourd’hui j’ai réussi à faire ma réservation en gare de Cluj mais par exemple à Iasi ou Oradea on ne pouvais pas me la faire, j’ai donc pris le train sans réservation ( les contrôleurs sont compréhensifs) mais on a d^^^^^^u passer des moments du trajet debout.

Je pense que à Bucarest ou les villes de Transylvanie ils ont plus à faire à Interrail mais dans les autres villes avec peu de touristes c’est plus compliqué.

So what did they say in Oradea then? They did not know what to do? Since in principle, all ticket offices that can sell reservations can sell reservations for Interrail. If they don't know what to do, you could suggest they ask their helpdesk…