Scripts must have a Main function

Unlike JavaScript that executes in a browser, Microsoft Advertising scripts must define a main() function. The main function is the script's entry point and contains the program logic you want to execute. The script may also define custom functions. For example, the following script defines a main function that calls a custom function named getAllCampaigns.

function main() {
    getAllCampaigns();
}

function getAllCampaigns() {
  var campaigns = AdsApp.campaigns().get();
    while(campaigns.hasNext()){
        var campaign = campaigns.next();
    }
}