Deskera Accounting Error when adding a sub field

Hi everyone. I am having a problem with my Deskera Accounting application.
An error pop-upped that says “Field is blank or data already present” (attached below) whenever I try to add sub field.
What does this error means?
I already entered data to the field. Is there something wrong with whatever I am trying to enter?
If anyone can help,
Thanks!

