There are two possible errors relating to interactions not responding:
This flow is missing a
Interaction Responseblock. Please add one to customize this message.
or
This application did not respond.
TL;DR: In most cases, the error can be fixed switching from a send message block to a interaction reply block.
These errors happen when you have a flow triggered by a Discord interaction (e.g. slash command, modal, button, etc.) but you aren’t actually responding to that interaction. Discord requires that all interactions (slash commands, modal submissions, button presses, etc.) have an interaction response of some kind.
This most commonly happens with slash commands, when you use a Send Message block rather than a Text Reply to Interaction block. This causes the interaction to get no response, although your flow will otherwise work as expected.
In cases where you want to use a standard message send (like creating a giveaway), you can add a basic “Done!” or “Giveaway created!” interaction reply. You can even tag it as Ephemeral so it only shows up to the person who ran the command.