At the time of deleting a employee ERP next showing a pop up that employee link with salary structure.
The Salary Structuer is linked with that employee so you have to first delete the salary structuer then you can delete the Employee.
1.You have to set the permission for delete in that Doctype then you give option to delete.
2.Got to the Salary Structuer list
3.select that employee salary structuer & delete it.
4.Select that employee for delete.
In Case You are using the Demo.erpnext.com it is not allow you to delete.