Shift+Ñ seem to be the default keyboard shortcut for stop tuplet creation. But in Preferences>Keyboard shortcut the box appears empty.
If I try to insert Shift + Ñ, I get a warning:
“! Secuencia de teclas ya asignada a: NoteImput.EndTupletRun” (! Sequence of keys already assigned to: NoteImput.EndTupletRun) which seems to be precisely the function that I try to access.
As you will have seen from that thread, I’ve made some changes to the warning messages that will show you exactly the same name as that which appears in the list to the left, which should make it easier to see what’s actually going on. However, I don’t think you need to worry about Dorico is telling you that overriding a shortcut with itself, since it will end up making no difference to what shortcuts actually work.