=============== Solution thanks to acw1668 =================

Updated Excel_Frame 
1) Added width=800, height=300 
2) Added sticky="nsew" 
3) Added Excel_Frame.pack_propagate(0)


# Create Excel_Frame for TreeView Excelsheet
Excel_Frame = ttk.Frame(Main_Frame, width=800, height=300)
Excel_Frame.grid(row=0, column=1, rowspan=20, sticky="nsew")

treeScroll_x = ttk.Scrollbar(Excel_Frame, orient="horizontal")
treeScroll_y = ttk.Scrollbar(Excel_Frame, orient="vertical")

treeScroll_x.pack(side="bottom", fill="x")
treeScroll_y.pack(side="right", fill="y")

treeview = ttk.Treeview(Excel_Frame, show="headings", xscrollcommand=treeScroll_x.set, yscrollcommand=treeScroll_y.set)

treeview.pack(side="left", fill="both", expand=True, padx=5, pady=5)

treeScroll_x.config(command=treeview.xview)
treeScroll_y.config(command=treeview.yview)

Excel_Frame.pack_propagate(0)