Contact SNCF TER NOMAD customer service directly. That seems the best option to me.

If you have an answer, you can always share it with the community here to help others.

ONLY now mandatory (or they say so) on weekends and to/fro Paris, la ville lumiëre. There is really no need at all to worry about that many monthes advance-I guess they are not even possible. Though they may lay on more restrictions if covid strikes again or on sunny summerdays. You did not state any travelday, In Paris on the relevant platfs are now gates-to open with QR code on RES-slip, but the other way you can just board-I assume you stay there a short time? Just ask/arrange at counter-brush up your knowledge of French. Fair chance they just wave you on and tell its all not needed.

I have contacted SNCF Normandy customer service and they provide the following information.

All trains K (KRONO and KRONO + ) have a mandatory reservation.

Thank you for your replies, if SCNF advise that seat reservations are mandatory and they cannot be confirmed on eurail.com or the app, then what is the alternative. 

Dates are 24-27 May, passengers do not arrive into France until 5 days prior to travel date. 

thank you 

That is not what they state on their website.

Hi Nikki, thanks for your question. I am sorry for the late response!
This might have been an availability issue, as most trains in France need to be reserved well in advance. 
Please also let me explain that the Pass and seat reservations are independent and, as such, reservations will not show in the app. You will receive them by email or by post-depending on the type of reservation offered- and you will need to show the Pass and the seat reservations during controls.

I hope this helps! Please let me know if you have any questions.

Kind regards,

Annie
 

@Annie. 

This was not an availability issue. These TER reservations between Paris and Normandy have never been bookable via Eurail/Interrail.

Maybe you could look into when this could be implemented?

Side note: the app claims these reservations cost €12, whereas in reality they cost €1.50 (SNCF ticket office or ticket machine).