How do I assign an existing custom IAM role as execution role for a lambda

I am trying to assign an existing role created via AWS console, but I face an error Resource handler returned message: "The role defined for the function cannot be assumed by Lambda.

const customRole = Role.fromRoleName(this,"MyAPIRole", "EXISTINGEXECUTIONROLENAME")

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this.lambdaFunction = new Function(this, 'CrispSkillUpdateBot', {
      functionName: ,
      description: ,
      code: ,
      handler: 'xyz.lambda_handler',
      role: customRole,
      memorySize: 512,
      timeout: Duration.seconds(30),
      runtime: Runtime.PYTHON_3_8,
    });

I face an error Resource handler returned message: "The role defined for the function cannot be assumed by Lambda.

This is not a CDK-related problem necessarily. Your role needs a Trust Policy that allows AWS Lambda service principal to assume the role.

Check out the documentation. The required trust policy is:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

If your custom role was also created in CDK, you could use the following:

const customRole = new Role(this, 'Role', {
    // ...
    assumedBy: new ServicePrincipal('lambda.amazonaws.com'),
});