Вам необходимо отделить код, создающий компонент Treeview, от кода, который вставляет в него новые элементы, в противном случае вы создаете новый компонент Treeview с одним элементом в нем для каждой строки в ваших результатах.
def EmployeeScreen():
mycursor.execute("""SELECT ID, Username, Processor, MotherBoard, Memory, Storage, Graphics, PowerSupply, TowerCase, Price FROM PendingOrders""")
results = mycursor.fetchall()
PendingOrdersTree = ttk.Treeview(Employee, selectmode = 'browse')
PendingOrdersTree.grid(row = 3, column = 2, pady = 25, padx = 15)
PendingOrdersTree["columns"] = ("1", "2", "3", "4", "5", "6", "7", "8", "9", "10")
PendingOrdersTree['show'] = 'headings'
PendingOrdersTree.column("1", width = 75, anchor = 'c')
PendingOrdersTree.column("2", width = 75, anchor = 'c')
PendingOrdersTree.column("3", width = 75, anchor = 'c')
PendingOrdersTree.column("4", width = 75, anchor = 'c')
PendingOrdersTree.column("5", width = 75, anchor = 'c')
PendingOrdersTree.column("6", width = 75, anchor = 'c')
PendingOrdersTree.column("7", width = 75, anchor = 'c')
PendingOrdersTree.column("8", width = 75, anchor = 'c')
PendingOrdersTree.column("9", width = 75, anchor = 'c')
PendingOrdersTree.column("10", width = 75, anchor = 'c')
PendingOrdersTree.heading("1", text = "ID")
PendingOrdersTree.heading("2", text = "Username")
PendingOrdersTree.heading("3", text = "CPU")
PendingOrdersTree.heading("4", text = "Motherboard")
PendingOrdersTree.heading("5", text = "RAM")
PendingOrdersTree.heading("6", text = "Storage")
PendingOrdersTree.heading("7", text = "GPU")
PendingOrdersTree.heading("8", text = "PSU")
PendingOrdersTree.heading("9", text = "Case")
PendingOrdersTree.heading("10", text = "Price")
for i in results:
PendingOrdersTree.insert("", 'end', text = "L1", values = (i[0], i[1], i[2], i[3], i[4], i[5], i[6], i[7], i[8], i[9]))