Hi there, @jaimeisaac… I’m not sure I understand your post, but it seems like you shouldn’t be focused on the Only when condition when you haven’t defined the thing you want to change. Are you trying to change a thing in the cliente data type where the thing’s nombre field is the same as the value that has been entered into the Input nombre cliente input? If so, I believe the thing to change would be Search for clientes:first item with a constraint on the search of nombre = Input nombre cliente's value. The search would either find a thing to change or it wouldn’t, so I’m not sure why you would need an Only when condition. If you do need the condition, you can use the same search I mentioned above, and the condition would be Search for cliente's:count > 0.
If I have missed the mark here, share some more details/screenshots, and I am sure someone will be able to help you out.
Best…
Mike