You can't make reservations in the app. I don't know what's happening on the website. Maybe restart your phone ?

I have already done that. May need to uninstall and reinstall the app. But will I lose my information?

No need to reinstall the app if you have a problem with making reservations. The app is not involved in that.

What train are you trying to book? (Route, date, departure time) There are often better and cheaper ways to book than on eurail.com.

Falmouth to Bournemouth 4th July 09:18.

It is already on my pass, just trying to make a reservation.

1. It's too early for that date.
2. Please check British timetables on www.nationalrail.co.uk. The rail planner app is not reliable at all.
3. Eurail can't book reservations for British trains.
4. The first train probably doesn't have reservations. The rest can probably be booked via GWR, when reservations are released. There's no rush though.

In addition there are no mandatory reservations for almost all trains in the UK, although reservations for mainline/express trains is always recommended. 

Thanks for that, I do use GWR, very handy tool, just for reservations. The website not very reliable though, as I am in NZ, I cannot use their app.