Hi Juan, if you have added the “legend” object in the parent “styles” within your JSON configuration, you should be able to see the legend in the EO Browser (Sentinel Hub EO Browser). Log in and, in the search tab, choose the theme based on your configuration (from the dropdown menu). You should be able to see the legend as in attached example.
If the legend image is shown, you have configured the legend correctly.
As for displaying the legend in your WMS portal please provide some more details such as the specific url that the portal is using when trying to load the legend. Example:
https://services.sentinel-hub.com/ogc/wms/{instanceID}
?service=WMS&request=GetLegendGraphic&layer=MOISTURE-INDEX&height=200&width=100