Hello, @Mark29,
Welcome to our community!
That should be for the code cell with the exercise code #UNQ_C5, right? The default code structure for that cell is that it (1) sets the seed, (2) define model_r and (3) compile model_r. It does not require any function named build_and_train_model, and function of such name is not required anywhere in the Machine Learning Specialization Course 2 Week 3 assignment.
Do you think that’s your own creation to have build_and_train_model? If so, in terms of passing the assignment, it might just be the quickest fix for you to fall back to the original structure and complete the step (2) and (3) above. If the original structure was lost, you may follow these steps to get a new copy of the notebook while keeping your current one so that you could move your work over.
If the function was already there and you were just completing it, I wonder if you might be referencing another assignment because as I said in my 2nd paragraph, it only defines and compile, so it does not train (as implied by the name build_and_train_model.
Let us know your feedback or how it goes.
Cheers,
Raymond