Within the Flow Builder, you can mostly only get validation errors, that is, either some of the required fields of each Step of your flow are empty (not filled in), or they’re filled in incorrectly (invalid phone number, invalid email, etc.)
If a Step has an error, a red bubble will be displayed on top of it. If you click on the Step with the error, you’ll see that Flow Builder marked the exact error in red. Don’t worry! Flow Builder won’t allow you to publish a communications flow that contains any errors.