How to fix "The OAuth client was not found" error from a Bing Ads script - google-oauth

Question

We've got scripts on Bing to automatically adjust ad bids based on ad performance and client goals, which are stored in a Google spreadsheet.
We had a contractor set this up initially, and it worked. But I guess that the contractor was using a temp Google account and when it went away the bidders stopped working. Because it did work before, it's likely a configuration error on my part that's breaking it now, but the contractor pointed us to the steps I was already following to no avail (https://learn.microsoft.com/en-us/advertising/scripts/examples/authenticating-with-google-services#option2).
Stuff already tried
double checked for errant whitespace around the client ID and client secret
created new client secrets
created new client IDs
made sure that the project name, application name, and OAuth client id name were all the same
created whole new projects from scratch (configured to match the article cited above) to see if that would kick something loose
tried a different token URL (https://oauth2.googleapis.com/token) that appears in the client_secret JSON downloaded from Google
function main() {
const credentials = {
accessToken: '',
client_id: 'REDACTED.apps.googleusercontent.com', // from Google developer console
client_secret: 'REDACTED', // from Google developer console
refresh_token: 'REDACTED' // created at https://developers.google.com/oauthplayground
};
var access_token = '';
if (credentials.accessToken) {
access_token = credentials.accessToken;
}
var tokenResponse = UrlFetchApp.fetch('https://www.googleapis.com/oauth2/v4/token', { method: 'post', contentType: 'application/x-www-form-urlencoded', muteHttpExceptions: true, payload: { client_id: credentials.clientId, client_secret: credentials.clientSecret, refresh_token: credentials.refreshToken, grant_type: 'refresh_token' } });
var responseCode = tokenResponse.getResponseCode();
var responseText = tokenResponse.getContentText();
if (responseCode >= 200 && responseCode <= 299) {
access_token = JSON.parse(responseText)['access_token'];
}
throw responseText;
// use the access token to get client targets from the spreadsheet
A JSON encoded access token is the expected response, but instead, we get HTTP 400 with the message "The OAuth client was not found."
Manually creating an access token on the OAuth playground (https://developers.google.com/oauthplayground) works as a stopgap, but this should work. This has worked. :P

Answer 1

The fix in this case switching the Application Type on console.developers.google.com > Credentials > OAuth consent screen to Internal instead of Public.
That wasn't in the steps provided by Microsoft, and I'm not sure if that will have implications down the road, but at least we're off the manual process for now.