Hi @Philippe_B! The « Speak on a speaker » action from Gladys generates an audio file and asks the speaker to play it, which an Echo cannot do from an integration. Your Echo appears in the list because the integration also publishes it as a music player, but this action will not work with it.
In your scene, replace it with the integration action « Make an Echo speak »: choose your Echo (oxo), enter your text, and optionally the volume. It’s the same as the « Test: make an Echo speak » button that works for you 