Can I book a firts class seat with a second class pass?

Often there is nothing stopping you buying 1st class reservations but in most cases if you only have a 2nd class pass you are not entitled to travel in 1st class even with 1st class reservations, doing so could lead you to be removed or penalised for travelling without the correct ticket.

It does depend on the rail company involved though so without more details that is the general answer that applies to most situations.

Sometimes it's possible, sometimes not. In most cases, you'll need to pay extra for the upgrade. I believe Happyrail supposes you have a 1st class pass when you book a 1st class reservation.

For more details, route, date and departure time are needed.

Thanks, the route I need is Madrid to Narbonne on August 15th and it is operated by Renfe, do you know if this company let it do it that way?

That should be possible.

When checking renfe.com, I see there are still 61 seats in 2nd class available (here one of the carriages with free seats):

But at the same time, I don't see any availability via happyrail.com, raileurope.com or interrail.eu. Not even in 1st class:

​@Eurail Community Moderator is there a new problem with booking the international AVEs? I can't even find seats 3 months in advance.

When checking renfe.com, I see there are still 61 seats in 2nd class available (here one of the carriages with free seats):

But at the same time, I don't see any availability via happyrail.com, raileurope.com or interrail.eu. Not even in 1st class:

​@Eurail Community Moderator is there a new problem with booking the international AVEs? I can't even find seats 3 months in advance.

Finally yesterday night, I could buy 2 seats in first class (maybe the last to seats availables) but I don't want to have problems the travelling day...

When checking renfe.com, I see there are still 61 seats in 2nd class available (here one of the carriages with free seats):

But at the same time, I don't see any availability via happyrail.com, raileurope.com or interrail.eu. Not even in 1st class:

​@Eurail Community Moderator is there a new problem with booking the international AVEs? I can't even find seats 3 months in advance.

I think ​@Schelte reported that a few weeks ago. There might be a new very restrictive passholder quota on these trains.

Maybe. But there's definitely a problem on interrail.eu. 3 months in advance works on Rail Europe, but not via Interrail.