Apps Script: Script function not found

Apps Script was asked to run a function by name and no function with that name exists in the project. The name after the colon is what it looked for.

For web apps the name is doGet or doPost: Google's web app guide requires a script to define one of these to be deployed as a web app, and a GET to a deployment without doGet (or a POST without doPost) produces this message. For triggers, menus, buttons and drawings, the name is whatever was assigned to them; renaming or deleting that function leaves the trigger pointing at nothing.

Nothing is wrong with the function's body. The caller and the code simply disagree about a name.

function doGet(e) {
  return ContentService.createTextOutput('ok');
}

function doPost(e) {
  const data = JSON.parse(e.postData.contents);
  return ContentService.createTextOutput(JSON.stringify({ received: true }))
    .setMimeType(ContentService.MimeType.JSON);
}

Treat function names used by triggers, menus and deployments as a public interface: do not rename them casually, and keep a comment listing what calls each one. After changing a web app, always create a new deployment version and test the deployed URL, not just the editor.