How do you tell alexa to activate the stop mode?
I have configured it like you, in the alexa app I see the mode stop, up and down. I can select it and I see the change in the openhab log. But I am not able to tell alexa to do so 
Edit:
Now I got it 
Your example is an english one. In my case alexa understand “stopp” (german) and your example uses stop.
For all german ones, here my setup + commands:
Group gEins "eins" {alexa="Endpoint.EXTERIOR_BLIND"}
String Rollo_WZ_klein_stopp "halt" (gEins) {alexa="ModeController.mode" [supportedModes="STOP=stopp,UP=auf,DOWN=zu", autoupdate="false"]}
Rollershutter Rollo_WZ_klein "eins Rollo" (gEins) {alexa="RangeController.rangeValue" [category="EXTERIOR_BLIND", friendlyNames="@Setting.Opening",
supportedRange="0:100:10", unitOfMeasure="Percent", actionMappings="Close=100,Open=0,Lower=(+10),Raise=(-10)",
stateMappings="Closed=100,Open=0:100"],channel="loxone:miniserver:.." }
- schliesse eins (close eins) → full close
- oeffne eins (open eins) → full open
- setze eins auf xx% (set eins auf xx %) → moves to % value
- fahre eins hoch (raise eins) → moves up by 10%
- fahre eins runter (lower eins) → moves down by 10%
- eins stopp (oder: setze eins auf stopp) (set eins to stopp) → STOP mode is selected