That is a DIRECT train (TGV runs about ev hour here-in 2 hrs) or a rare one (but lower price) with a change somewhere? Probably not. 2nd leg would then be a local TER=without even RES possible.

Most likely explan: on these TGV there is a small quota for passholders+cost then 10€. After this is filled seats-as long as there are-go for 20€. This app/EU canNOT handle the latter. If you are really hell-bent on doing the train company 

If you give the exact details of the journey you are trying to make a reservation for, you will get some more advice. 

Im trying to book from Tours to bordeax and 1 leg is not in pass network do i have to buy a ticket even with a global pass ?

Any trains with the designation OGV are operated by Ouigo, and not covered by the pass.

Any journeys on TER or TGV are covered by the pass, and you will need a reservation on the TGV portion.

Generally, you will need to get a 5 minute unreserved local train (TER) to St Pierre des Corps, just outside the centre of Tours. (Or you can walk/ taxi/ bus if that’s more convenient for your accommodation.) The main TGV line is there, and you can get a fast train to Bordeaux.

Make your reservation on the TGV here: https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish