Hi Sylvian
Sorry about this issue. You are correct that in order to create a Shortlist from an existing web map, including the option to use existing point data in that web map as places in your Shortlist, you have to create the app from the map by choosing Shortlist from the Configurable app gallery. Like you say, if you just want to create a Shortlist from scratch without using an existing web map, you can launch the Shortlist Builder from the Shortlist page on the Story Maps website (or search for the Shortlist app item page in ArcGIS Online and launch it from there).
Looks like you are using ArcGIS Online in French. The current Story Map Shortlist beta isn't translated into all the languages supported in ArcGIS Online yet, and so it doesn't appear in the Configurable app gallery for non-English languages yet. If you would like to use it in English, you may be able to change the language setting in your ArcGIS Online account profile to English, which case you should see Shortlist in the English language Configurable app gallery. Or, if you are using an Organizational subscription, you could ask your admin to add the English language version of the app into the default group used to populate the app gallery. But see the next paragraph..
In the upcoming ArcGIS Online update (currently planned for March 1st) Story Map Shortlist is being translated into all supported ArcGIS Online languages. It will also automatically appear in the Configurable app gallery for English, French, German and Spanish languages, so you will see it. It won't be appearing in the Configurable app gallery for other languages until it is out of beta.
I think the above explains your situation. Just to complete this post for other people who land on this thread, there are two other reasons why Story Map Shortlist may not be in your App Gallery: 1) You may be using Portal for ArcGIS, not ArcGIS Online. Shortlist isn't available in Portal yet because it is still in beta, but should be available later in the year once we release our final version. 2) Your ArcGIS Online Organization's Configurable app gallery might be customized in your enterprise to remove that app for some reason. Let your admin know you want it or if you don't want to create a Shortlist from a web map, create your Shortlist from the Story Maps website.
Rupert