What kind of pass do you have (global pass? one country pass?) and what is the journey you are trying to add to your pass? Could you please post the train details from My Trip?

In case you have a global pass and the last travel day is for a journey back to your country of residence:

Please check ASAP (before midnight today!) under My Pass how many Outbound/Inbound you have used. If it says 2/2, then check in My Trip on which days they are used: you will see an icon "Out/In” to the right of the date. That means that you travel in your country of residence on that day. That is only possible on max. 2 travel days.

In case you see the "Out/In” icon on a travel day where you would not expect it, please post the train details and remove the journey from My Pass. There's no need to activate travel days in advance and removing from My Pass is only possible before a travel day starts. After that, we can hopefully figure out what's wrong.

Interrail France Pass
4 days within 1 month
Paris - Basel SBB

OK, I think I see the problem. Basel SBB is not supposed to be included in the one country pass for France:

One the one hand, that is logical because Basel is in Switzerland, but on the other hand, tickets from France to Basel SBB are sold as French domestic (!) tickets, since Basel SBB is the tariff border point between Swiss and French railways. (Except that the Interrail reservation from France to Basel is much more expensive than a French domestic one.)

And if Basel SBB is not included, then it is strange that a journey from Basel to Paris was accepted by the app. Are you using a TGV from/to Basel in both directions?

What you could an do is add this last train manually. Look up this journey in the planner, scroll to the bottom of the results and tap on "Add it manually”. Then change the country of Basel SBB to France, check the departure/arrival times and add the train number.

Genius! Many thanks! It worked manually.
Great!

It USED to be the case that Basel SBB had a special section (called French side or so, of course all in French) and this was to make connections even separate and to last longer-as it dates from the times that there were still bordercontrols. Perhaps this name is still hidden somewhere in the French burocracy of their lists of station names.

It seems-as this has been reported a few times here now, that this is yet another bug of the ystem in that it is unable to recognise that a station may belong to 2 countries. Maybe we can if people start travel more, get the same case for Kufstein, Passau, Salzburg, all both German and Austrian tariff points.