Paris - Milan SNCF TGV are included (with a hefty reservation fee) Trenitalia Frecciarossa are not included.

You haven’t said what date you are looking for.

Nor the departure time or what the error message was, so we can only guess and not check or test.

Thank you so much for your help. I am trying to book on 6/18/2023 the 9:44 TGV9245 train from Paris Gare de Lyon to Milano P.Garibaldi. The error is “One of your legs could not be booked” and below that is says “one of the legs of your journey is not available any more.” The problem is I have tried different routes that day, different trains, first class, second class, and I still can’t get anything to book. Even tried a month ago so I don’t think it was that the train was sold out, but I am not sure of that. I have 6 people in my family so maybe that’s an issue? We do have the Eurail passes that cover first class seats. Also, it is happening on the page where I agree to terms and conditions and then it should “proceed to payment.”

Thanks so much for your help. I’d hate to mess this up so we couldn’t get any train that day to Milan. 

I'm able to book that train so I guess it is the 6-people issue (it works for max 4 AFAIK ?)

You can call SNCF though :

https://www.sncf.com/en/customer-service/contact-us/telephone

#85 for English, no booking fees and reservations sent via e-mail

Oh my goodness...I figured it out. It WAS the 6 tickets I was trying to book. I split them up and it worked great. Thank you for all of your help! Well, now I know better for next time! Man, this system is easy but only if you know what you are doing! 

The 6 people may be the problem. For 5 I can get to the payment page but not for 6. IIRC booking SNCF trains this way only works for groups up to 5.

You can probably book them in 2 groups, e.g. 2+4, but you could be in totally different carriages.

You can also book by phone via NS or SNCF. There are no booking fees this way but of course the phone call is not free of charge.