It appears that you made a mistake while following the steps to create a neural network using the "nntool" application. I suggest you follow the steps again after closing the application and restarting it.
You could also upgrade MATLAB to R2024b. The newer version has significant enhancements in the Deep Learning Toolbox, and the added functionality would be a major advantage since this issue is not reported in MATLAB R2024b due to the removal of the "nntool" function for starting the Neural Network Manager application.
To read more about the Deep Learning Toolbox in MATLAB R2024b, refer to the following documentation: https://www.mathworks.com/help/deeplearning/index.html
