computing the math formula on the mask (google earth engine)

I am about to calculate Chorophyll-a in the water bodies in one region, as I outlined above. I have created a mask, with water=1, land=0(transparent）. And I want to calculate quality formula (NDCI, refer to normalized difference chl-a index) over the mask I created in the last step. Here are my code.

function maskS2clouds(image) {
       var qa = image.select('QA60')
        var cloudBitMask = 1 << 10;
        var cirrusBitMask = 1 << 11;
 var mask = qa.bitwiseAnd(cloudBitMask).eq(0).and(
         qa.bitwiseAnd(cirrusBitMask).eq(0))
 return image.updateMask(mask).divide(10000)
  .select("B.*")
  .copyProperties(image, ["system:time_start"])
   }

  var tiles = ['29UNV']
   var collection = ee.ImageCollection("COPERNICUS/S2_SR")
   .filterDate('2020-01-01', '2020-12-31')
    .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
   .filter(ee.Filter.inList('MGRS_TILE', tiles))
   print(collection)
   var minmin = collection.map(maskS2clouds)
   print(minmin)
    var calndwi = function(image){
     //water mask
     var NDWI = image.normalizedDifference(['B3', 'B8']).rename('NDWI');
     return image.addBands(NDWI).updateMask(NDWI.gt(0));
    };
   print(minmin.map(calndwi));
  //Add NDWI to the clipped image collection
   var withNDWI = minmin.map(calndwi).select('NDWI');
   print("NDWI collection", withNDWI);
  var bb = withNDWI.first();
  Map.addLayer(bb,{},'ss');
   var addNDCI = function(image) {
  var ndci = image.normalizedDifference(['B5', 'B4']).rename('NDCI');
    return image.addBands(ndci);
   };
   var withNDCI = minmin.map(addNDCI).select('NDCI');
   print("NDCI collection", withNDCI);
   var MASK = function(image) {
  var mask = bb.mask(image);
     return image.addBands(mask);
   };
  var maskk = withNDCI.map(MASK).select('mask');
  print(maskk)**

and it give me the bug like ImageCollection (Error) Error in map(ID=20200106T114451_20200106T114531_T29UNV):Image.select: Pattern 'mask' did not match any bands.what should I do? thanks a million

The maskk object does not contain any bands named mask, because your MASK function does not create or add any bands with that name.

What your code does, as you've currently written it, is this:

   var MASK = function(image) {
  // Apply a mask over the 'bb' image.
  var mask = bb.mask(image);
     // return 'image' (which was the 'mask' parameter above), 
     // with ALL bands from the object 'mask', which is now a 'masked' version of bb. 
     // since mask = bb.mask(image), all the bands from bb will be added. 
     return image.addBands(mask);
   };

var maskk = withNDCI
  // Map the 'MASK' function over the 'withNDCI' collection
  .map(MASK)
  // Attempt to select a band named 'mask' (which does not exist).
  .select('mask');

I'm not sure what you're looking for when you try to select the mask 'band' - I assume what you want is the masked NCDI image. That's essentially what you have already - but the band names of the 'maskk' object are "NDWI" and "NDCI", since it is derived from the bb, and those are the bands that bb contains. There is no band named "mask".