Where are you trying to book for?

Generally software not offering the next option means one of the info boxes has invalid info or you haven’t accepted T&Cs. 

It can be frustrating to diagnose.

I booked in google chrome in both my phone and PC and its doesnt work. Even incognito mode that some in this community suggest, its not work too. still not respond anything

So what are you trying to book? (route, date, departure time)

Depending on that information people can advise other ways to book. Possibly also cheaper.

We need more info before we can help. Where and when are you trying to book for? Are you on the Interrail reservation tool, have you tried any of the alternative sites recommended on the community? 

I try to book 2 destinations
1. PARIS EST (France) (06:55 AM) to KARLSRUHE HBF (Germany) (09:26 AM) on Sunday, October 23, 2022
2. SPIEZ (Switzerland)(12:05 PM) to MILANO CENTRALE (Italy)(02:40 PM) on Tuesday, October 25, 2022

and yes, im using eurail reservation tool on this website https://www.eurail.com/en/book-reservations#/login but didnt try to use another website

Via Interrail/Eurail is the only online possibility. Alternatives include calling SNCF, NS or NMBS/SNCB.

Did you check this checkbox?

You can book that train online on tickets.oebb.at, without any booking fee (so 2€ cheaper than Eurail/Interrail). Do NOT use "Seat only” but add the Interrail/Eurail discount and then look for normal tickets.

@2: check on sbb.com if there are not again some works along the way which needs BUStitution needed for some part-as it is also now. I dk how long this will take (on the Italian part-which today is also again on strike). Plus that, really, do not worry too much about it now-just do it in person after arr in the Swiss-not necessarily Spiez, its 11€ extra for 2nd.

You can also save money-and have more options-but take more time+changes as INTern TGV direct FRance-Germany cost much more as domestic France (till Strasbourg)-then local/regional small shuttle train over Rhinebridge.