Reservation Paris- Nice says ''one leg is unavailable'' but doesn't say which one.

On which date are you trying to reserve?

Traveling from Paris Gare de Leon to Nice Ville May 31st at 10:30 am

I have also tried to book on travel.b-europe.com, but it requires a pass cover number which I do not have as I only have a mobile pass on the app.

Change in Marseille to TER train?

In THAT case: again, for the 12345th time (and whilst I actually now stay on that area around): TER trains in FRance are Local=regional and canNOT even be REServed. IF you use the planner from bahn.de (german rails, works for nearly all of EUR) it very clearly shows what trains need what or not. 

IN France on SNCF ONLY the TGV=superfast hi-speed and a few IC trains (and the overnight trains that have revived on a very few routes) have mandatory REServ. Funny is I´ve never ever heard someone asking about these IC-trains (old Corail stock with loco) which run on some major lines.

The major probs reported again+even more on this forum are for the TGV that run overtheborder, mostly ESpana. Hard to get and superexpensive compared to changing to local trains-but the cost for direct TGV to Italy is even higher.

You can request a pass Covernumber from Eurail Staff here in the community :) @Magdalena for example. 
 

Also, if you're in France, the easiest is often to book at the ticket office. Added advantage is that you don't pay any booking fees.

The SNCF ticket can machines also issue Interrail reservations, but for domestic trains only.