This is a TER regional train. Generally those trains do not have seat reservations but the ones run by the Normandy region (NOMAD trains) are an exception.
They require a 1.50€ reservation, available at any SNCF ticket machine (not anywhere else). Simply make it when you arrive in the country, availability won't be a problem.
It may be a bit annoying but it is the Normandy's regional goverment decision, not Eurail's. Frankly don't worry too much, you'll easily find seats even on the day itself.
Please also note that some trains can only be booked 60 days in advance, not the case here but just wanted to add.
Btw when you mean "your reservation" please note that nobody here works for Interrail/Eurail (except those with the logo next to their username). We are here helping people during our free time.