To get pixel information from an image, you will have to use image.reduceRegion(s). That will return a dictionary or feature collection providing you with the pixel information of the bands of the input image. There can only be one image as input.
If you want the mean soil moisture over the whole study period defined in filterDate(), you probably best make a mean composite image of your image collection (which you confusingly call 'image') and apply reduceRegion on that image:
// Get the mean from the dictionary and print it.
var meanDict = soilmoisture.mean().reduceRegion(ee.Reducer.mean(), Zutendaal, scale);
print('Gemiddelde bodemvochtgehalte studie gebied voor het hele jaar (mm)', meanDict);
It's probably more interesting to get the mean soil moisture of each image in your study period/time. Therefore, you will have to map over the image collection and return every output dictionary as property to the image:
// Take average
var imageCollection = imageCollection.map(function(image){
return ee.Image(image.setMulti(image.reduceRegion(ee.Reducer.mean(), Zutendaal, scale)));
});
You can then print a list of soil moistures. Also, I added a chart of the soil moisture inside the study area over the study period:
// Get the mean from the dictionary and print it.
var means = imageCollection.aggregate_array('ssm');
print('Gemiddelde bodemvochtgehalte voor elke afbeelding (mm)', means);
// put in a chart
print(ui.Chart.image.seriesByRegion(imageCollection, Zutendaal, ee.Reducer.mean(), 'ssm', scale));
Think about the scale you will need to use using reduce regions, as your images are on 0.25 arc degrees.
Link script
The line:
var val = ee.Number(data.get(['ndvi']))
needs to be changed to
var val = ee.Number(data.get('NDVI'))
since you have renamed 'ndvi' to 'NDVI' in your code.
Best Answer
In this question, you are asking to handle a histogram output, which is a e.Dictionary(). So you should cast the output to a dictionary and then use get('bucketMeans') to get the single outputs.
link code
A fixedHistogram reducer returns an array, see the answer I posted in your previous question.