GetLegendGraphic return a symbol instead of colormap ramp

Question

I'm trying to put inside an OpenLayers project the legend for an NDVI map. Previously I've created with QGIS 3.10 the SLD as indicated on the GeoServer's official documentation.

When I try to use GetLegendGraphic to see the legend's preview I see only the symbol of raster and not the legend. If I set the style as default for my raster layer I can use the link from GetLegendGraphic for see correcty my legend. But If I set "Associated" instead of "Dafault" I see again the raster's symbol.

Why I'm obligated to use "Default" for styling a raster?

EDIT

Here the WMS stylized(A) and here the WMS not stylized(B). The style name is NDVI.

https://gis.massimilianomoraca.it/geoserver/MassimilianoMoraca/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetLegendGraphic&FORMAT=image%2Fpng&LAYER=NDVI_Campania_20150807&SCALE=136494.42037699942

gives

it is possible to see the legend and is not possible to do the same with

https://gis.massimilianomoraca.it/geoserver/MassimilianoMoraca/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetLegendGraphic&FORMAT=image%2Fpng&LAYER=NDVI_Campania_20160712&SCALE=136494.42037699942

.

The layer A has NDVI as default style and for B I've used "Associated".

Answer 1

You need to specify the name of the style you would like the the legend to use if you don't want to use the default one - in the same way as you must to make it appear in the map.

So https://gis.massimilianomoraca.it/geoserver/MassimilianoMoraca/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetLegendGraphic&FORMAT=image%2Fpng&LAYER=NDVI_Campania_20160712&SCALE=136494.42037699942&style=NDVI will return: