DataRobot Prime needs validation data. You can run it on the 64% model, but you can also increase the size to 80, 90% or whatever your needs are by starting a new project, and setting the holdout to 0% allows you to get the maximum amount of data for training DR Prime models. It is normally recommended that at least 10% of the data be used for validation (very large datasets may use less), or the model results may be unstable.
The behavior you see is the correct way of the DataRobot Prime model. Compared to the teacher model, the Prime model may be trained only if it has extra data to train the student model. So 100% doesn't leave any data, that the teacher model hasn't seen. So does the 80% teacher model, because holdout data is unreachable during training. So as a result, your error means: "student model doesn't have data to train on"