Lambda layers unable to import module failing with Runtime.ImportModuleError in serverless architecture

Question

I want to implement Lambda Layers into my Serverless application. I plan to create three layers, each based on separate requirements.txt files located in separate directories. Unfortunately, I am having trouble implementing using the serverless configuration. The file tree looks like this:

app-directory/
├── lambda functions codes ...
├── layers/
│   ├── layer_A/
│   │   └── requirements.txt
│   ├── layer_B/
│   │   └── requirements.txt
│   └── layer_C/
│       └── requirements.txt
└── serverless.yml

According to the instructions

• https://www.serverless.com/framework/docs/providers/aws/guide/layers
• https://docs.aws.amazon.com/lambda/latest/dg/packaging-layers.html

during first attempt, I created the python venv using python3.9 -m venv python and then installed the requirements.txt dependencies using the pip install -r layers/layer_A/requirements.txt targetted in the python/lib/python3.9/site-packages/ folder, then zipped the python directory. I then created a layers property in the serverless.yml file where I included the following parameters:

layers:
  layerA:
    path: layers/layer_A
    compatibleRuntimes:
      - python3.9

  layerB:
    path: layers/layer_B
    compatibleRuntimes:
      - python3.9


  layerC:
    path: layers/layer_C
    compatibleRuntimes:
      - python3.9

The layer reference in the function is as follows:

  migrations:
    handler: src.migrations.lambda_handler
    layers:
      - !Ref LayerALambdaLayer
      - !Ref LayerBLambdaLayer
      - !Ref LayercLambdaLayer

After running deployment script containing sls deploy and sls invoke stage test --function migrations command which turns on migration lambda, I received an error:

{
    "errorMessage": "Unable to import module 'src.migrations.handler': No module named 'alembic'",
    "errorType": "Runtime.ImportModuleError",
    "requestId": "59a33572-9439-4590-b547-de6084e784b3",
    "stackTrace": []
}
Environment: darwin, node 20.8.0, framework 3.35.2 (local) 3.38.0v (global), plugin 7.0.5, SDK 4.4.0
Credentials: Local, "default" profile
Docs:        docs.serverless.com
Support:     forum.serverless.com
Bugs:        github.com/serverless/serverless/issues

Error:
Invoked function failed

For the second attempt, I created another directory only for requirements files. I have also created a bash script simply creating a python/ directory where I installed the dependencies using the pip3 command.

#!/bin/bash

set -e

LAYER_TARGET_DIR="layers"
LAYER_REQUIREMENTS_DIR="layers_requirements"
RUNTIME="python3.9"

for LAYER_PATH in $LAYER_TARGET_DIR/*/; do
  LAYER=$(basename $LAYER_PATH)

  printf "Processing $LAYER layer\n"
  if [ -f "$LAYER_PATH/python.zip" ]; then
    echo "Existing python.zip found in $LAYER layer. Deleting...\n"
    rm -rf "$LAYER_PATH/python.zip"
  fi

  printf "Installing dependencies for $LAYER layer\n"
  mkdir $LAYER_TARGET_DIR/$LAYER/python
  pip3 install -r $LAYER_REQUIREMENTS_DIR/$LAYER/requirements.txt -t $LAYER_TARGET_DIR/$LAYER/python/
  printf "Dependencies installed for $LAYER layer\n"
    
  printf "Zipping the python folder\n"
  cd $LAYER_TARGET_DIR/$LAYER
  zip -r9 python.zip python
  cd - > /dev/null

  rm -rf $LAYER_TARGET_DIR/$LAYER/python

  printf "$LAYER layer processing complete"
done

printf "\nAll layers processed\n--------------------------------------\n"

In the end directories looked like that:

├── lambda functions codes ...
├── layers_directories/
│   ├── layer_A/
│   │   └── requirements.txt
│   ├── layer_B/
│   │   └── requirements.txt
│   └── layer_C/
│       └── requirements.txt
├── layers/
│   ├── layer_A/
│   │   └── python.zip
│   ├── layer_B/
│   │   └── python.zip
│   └── layer_C/
│       └── python.zip
└── serverless.yml

This doesn't work either, received same error. I have checked twice, how does the directory structure looks like - same as required on AWS site.

Is something in my configuration incorrect? Am I forgetting something?