Metal kernel function not found

I am trying to use Metal Performance Shaders with Core Image.

Unfortunately, the filter code is in Objective C and I am using the bridge to make it work with the Swift project.

My-Project-Bridging-Header.h

#import "MyCustomFilter.h"

MyCustomFilter.h

#import <CoreImage/CoreImage.h>

@interface MyCustomFilter : CIFilter
......
@end

MyCustomFilter.m

#import "MyCustomFilter.h"
#import "MyCustomFilterKernel.h"

.......

MyCustomFilterKernel.h

#import <CoreImage/CoreImage.h>

@interface MyCustomFilterKernel : CIImageProcessorKernel

@end

MyCustomFilterKernel.m

#import "FTWaveformScopeKernel.h"
#import "FTWaveformScopeFilter.h"

#import <Metal/Metal.h>

    ...

+ (void) initialize {
 id<MTLFunction> customFunction = [defaultLibrary newFunctionWithName:@"custom_Function"];
 if (customFunction) {
  NSError* error;
  kCustomComputePipelineState = [kDevice newComputePipelineStateWithFunction: customFunction error:&error];
  if (error) {
    NSLog(@"error loading kernel function (custom_Function): %@", error);
  } 
 } else {
     NSLog(@"kernel function (custom_Function) not found");
 }

CustomFunction.metal

    #include <metal_stdlib>
    using namespace metal;
   
    ......

    kernel void
    custom_Function(texture2d<float, access::sample>        inTexture       [[texture(0)]],
                        texture2d<float, access::write>         outTexture      [[texture(1)]],
                        volatile device atomic_uint*            columnDataRed   [[buffer(0)]],
                        volatile device atomic_uint*            columnDataGreen [[buffer(1)]],
                        volatile device atomic_uint*            columnDataBlue  [[buffer(2)]],
                        sampler                                 wrapSampler     [[sampler(0)]],
                        uint2                                   gid                       
                        [[thread_position_in_grid]])
{

........

}

The error I get is (with the metal view being blank/transparent):

kernel function (custom_Function) not found

I've also done:

I am dealing with Xcode 12.x. The above code compiles and runs.

UPDATE

After that first reply, I followed the new Xcode 12 and later settings: https://developer.apple.com/videos/play/wwdc2020/10021/

1. Changed CustomFunction.metal in to CustomFunction.ci.metal

2. Added the first rule:

3. Added the third rule:

Now the project does not compile:

LLVM ERROR: Error opening 

    /Users/xxxxxx/Library/Developer/Xcode/DerivedData/xxxxxx-ggaphqixkeecbkewrntvhssdgnac/Build/Intermediates.noindex/xxxxxx.build/Debug-iphoneos/xxxxxx.build/DerivedSources/CustomFunction.ci.air': No such file or directory!
    Command RuleScriptExecution failed with a nonzero exit code


RuleScriptExecution /Users/xxxx/Library/Developer/Xcode/DerivedData/xxxx-ggaphqixkeecbkewrntvhssdgnac/Build/Products/Debug-iphoneos/xxxx\ xxxx.app/CustomFunction.ci.metallib /Users/xxxx/Library/Developer/Xcode/DerivedData/xxxx-xxxx-ggaphqixkeecbkewrntvhssdgnac/Build/Intermediates.noindex/xxxx-xxxx.build/Debug-iphoneos/xxxx-xxxx.build/DerivedSources/CustomFunction.ci.air normal undefined_arch (in target 'xxxx-xxxx' from project 'xxxx-xxxx')
    cd /Users/xxxx/Desktop/xxxx-xxxx\ CustomFunction
    /bin/sh -c xcrun\ metallib\ -cikernel\ \"\$\{INPUT_FILE_PATH\}\"\ -o\ \"\$\{SCRIPT_OUTPUT_FILE_0\}\"'
'

These are the two lines in text:

xcrun metallib -cikernel "${INPUT_FILE_PATH}" -o "${SCRIPT_OUTPUT_FILE_0}"

xcrun metal -c -I $MTL_HEADER_SEARCH_PATHS -fcikernel  "${INPUT_FILE_PATH}" -o "${SCRIPT_OUPUT_FILE_0}"

Since I had to type these looking at a screenshot, maybe I made a mistake interpreting? There is a double space after -fcikernel?