How to display a message below the sublist and above the save button on the expense report page and also how to add a text field on the same?

If you want to keep it simple, you can add a text field with a default value of "Enter xyz here" or whatever you want it to be. BUT, it will be the header or line level. I would suggest the classification area as it is easiest there. Without scripting, there isn't an option to place it in the area you indicated. That being said, you can do amazing things with workflows as well to make users follow particular processes.