API call from lambda function to AWS.SSM.putParameter() returns success, but leaves SSM.parameters unchanged

Hello all, AWS and node.js noob here. I am trying to save a code captured from URL parameters to my parameters store in the System Manager service, and later return a hashed value from various strings. My code returns the hash and I don't have any errors in the console, but when I test it with URL/?code=1234&challenge_code=5678, nothing changes in my parameter store. I already have an object named ebayUserCode in there of type string.

exports.handler = async (event) => {
    const crypto = require('crypto');
    const AWS = require('aws-sdk')
    let verificationToken = "FIXME";
    let endpoint = "FIXME";
    let code = "code";
    let challengeCode = "challenge_code";
    
    if (event.queryStringParameters && event.queryStringParameters.code) {
        console.log("Received name: " + event.queryStringParameters.code);
        code = event.queryStringParameters.code;
        var ssm = new AWS.SSM();
        var params = {
            Name: 'ebayUserCode', /* required */
            Value: code, /* required */
            Overwrite: true
            };
        ssm.putParameter(params, function(err, data) {
            if (err) console.log(err, err.stack); // an error occurred
            else     console.log(data);           // successful response
        });
    }
    if (event.queryStringParameters && event.queryStringParameters.challengeCode) {
        console.log("Received name: " + event.queryStringParameters.challengeCode);
        challengeCode = event.queryStringParameters.challengeCode;
    }
    
    const hash = crypto.createHash('sha256');
    hash.update(challengeCode);
    hash.update(verificationToken);
    hash.update(endpoint);
    const responseHash = hash.digest('hex');
    console.log(new Buffer.from(responseHash).toString());
    
    const response = {
        statusCode: 200,
        body: responseHash,
    };
    return response;
};

What am I doing wrong? Any help would be greatly appreciated. Thanks in advance!