Welcome to the community.
In this assignment, LSTM_cell is used as a global variable, and used by music_inference_model().
If any errors occur in coding, then, LSTM_cell has an incorrect information in there and keeps those.
So, during coding/debugging, it is better to reset LSTM_cell with this.
LSTM_cell = LSTM(n_a, return_state = True)
Here is the details.
But, of course, this does not mean a root cause of an initial error is fixed. But, once you reset LSTM_cell, then, you can tackle a root cause of an error if exist.
Hope this helps.
By the way, is this an assignment 3 in the first week of the Course 5 ?