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Can you show some code please?

Here you go

So in your lambda function, you need to specify the handler property:

const ruleReportsFunc = new aws.lambda.Function("ruleReportsFunc", {
    environment: {
       variables: {
          "RULE_REPORT_BUCKET": ruleReports.bucket,
       },
    },
    code: new pulumi.asset.AssetArchive({
       ".": new pulumi.asset.FileArchive("./app"),
    }),
    runtime: "nodejs12.x",
    role: ruleReportsFuncRole.arn,
    handler: "ruleReportsFunc.ruleReportsFunc" // <-- I think this is correct, but you'll need to check
 });

thanks a lot!