Apparently, I was overcomplicating it with the build request Also, need to provide triggerId instead of trigger name

The updated buildRequest:

const buildRequest = {
    name: `projects/${projectId}/locations/${triggerLocation}/triggers/${triggerId}`,
    source: {
        substitutions: buildSubstitutions
    }
}

Good idea to do a getBuildTrigger if facing a similar issue:

const { CloudBuildClient } = require('@google-cloud/cloudbuild');
const cloudBuildClient = new CloudBuildClient();

const request = {
   name: `projects/${projectId}/locations/${triggerLocation}/triggers/${triggerId}`
};

const response = await cloudBuildClient.getBuildTrigger(request);
console.log(response);