ZeroDivisionError: float division by zero

[DeaD_EyE]

There was more then one error.
A cleaned up version of your code:

Haven't tested much.

import math
from tkinter import (
    Tk,
    ttk,
    messagebox,
    StringVar,
    Label,
    Listbox,
    Scrollbar,
    Button,
    Frame,
    ACTIVE,
    ANCHOR,
    BOTH,
    END,
    YES,
    N,
    S,
    VERTICAL,
)

length = 9.92  # Länge
width = 10.5  # Breite
height = 2.34  # Höhe
A = length * width  # Nutzfläche = Deckenfläche : 104.16m²
A_f = 30.45  # Fensterfläche
A_w = (
    2 * (length * height) + 2 * (height * width) - A_f
)  # Wandfläche insgesamt: 65.12m²
V = length * width * height  # Volumen des Raumes: 243.73m³
AV = A / V  # Verhältnis Fläche zu Volumen: 0.43m²

# Constants for the geometry of the main window
APP_XPOS = 100
APP_YPOS = 100
APP_WIDTH = 300
APP_HEIGHT = 300
APP_TITLE = "Simple Applikation"
NORM_FONT = ("Arial", 16)
# Variables
items_for_listbox = ["Musik", "Sprache", "Vortrag", "Spr.+Vor.", "Sport"]


def abs_materialen(x):
    for n in range(x):
        if x == 1:
            return Wand * (0.03 + 0.03 + 0.03 + 0.04 + 0.05 + 0.06) / 6
        if x == 2:
            return Wand * (0.11 + 0.22 + 0.34 + 0.35 + 0.51 + 0.43) / 6
        if x == 3:
            return Wand * (0.02 + 0.02 + 0.03 + 0.03 + 0.04 + 0.06) / 6
        if x == 4:
            return Wand * (0.03 + 0.03 + 0.02 + 0.04 + 0.05 + 0.05) / 6
        if x == 11:
            return Boden * (0.04 + 0.04 + 0.05 + 0.06 + 0.06 + 0.06) / 6
        if x == 12:
            return Boden * (0.20 + 0.15 + 0.10 + 0.09 + 0.06 + 0.10) / 6
        if x == 13:
            return Boden * (0.15 + 0.08 + 0.07 + 0.06 + 0.06 + 0.06) / 6
        if x == 14:
            return Boden * (0.02 + 0.04 + 0.07 + 0.19 + 0.29 + 0.35) / 6
        if x == 10:
            return Fenster * (0.28 + 0.20 + 0.11 + 0.06 + 0.03 + 0.02) / 6


def abs_objekte(x):
    for n in range(x):
        if x == 1:
            return MP * (0.15 + 0.25 + 0.55 + 0.80 + 0.90 + 0.90) / 6
        if x == 2:
            return WP * (0.05 + 0.10 + 0.15 + 0.35 + 0.45 + 0.60) / 6
        if x == 3:
            return Stuhl * (0.05 + 0.15 + 0.20 + 0.10 + 0.03 + 0.03) / 6


Boden = 100
Decke = 100
Wand = 180
Fenster = 80

WP = 18  # Weibliche Person
MP = 19  # Männliche Person

Stuhl = WP + MP

abs_flaeche = (
    abs_materialen(4)
    + abs_materialen(11)
    + abs_objekte(1)
    + abs_objekte(2)
    + abs_objekte(3)
)


# ------------------------ popup -----------------------------------


def popupmsg(msg):
    popup = Tk()
    popup.wm_title("!")
    popup.iconbitmap("C:/Users/PC/Desktop/Icons/attention.ico")
    ttk.Label(popup, text=msg, font=NORM_FONT).pack(side="top", fill="x", pady=10)
    ttk.Button(popup, text="Okay", command=popup.destroy).pack()
    popup.mainloop()


# -----------------------------calculating -------------------------


# noinspection PyUnusedLocal
def select(event):
    global width
    # a = int(mylistbox.get(tk.ANCHOR))
    a = mylistbox.get(ANCHOR)
    width = curselection(a)
    selected_var.set(width)


def curselection(a):
    if a == "Musik":
        return 0.45 * math.log10(V) + 0.07

    elif a == "Sprache":
        return 0.37 * math.log10(V) - 0.14

    elif a == "Vortrag":
        return 0.32 * math.log10(V) - 0.17

    elif a == "Spr.+Vor.":
        return 0.26 * math.log10(V) - 0.14

    elif a == "Sport":
        return 0.75 * math.log10(V) - 1


def calculate():
    global width
    if mylistbox.get(ACTIVE):
        abs_fl_ges = (
            0.163 * V / width
        )  # gemäß Sabinesche Formel:T_soll = (0163*V)/A_abs
        absorber = abs_fl_ges - abs_flaeche
        result_var.set(absorber)
    elif width == 0:
        messagebox.showinfo("No selection")


# GUI
# App window
app_win = Tk()
app_win.title(APP_TITLE)
app_win.wm_geometry("%dx%d+%d+%d" % (APP_WIDTH, APP_HEIGHT, APP_XPOS, APP_YPOS))

# Main container embeded in the app window
# Container for all of the app widgets
main_frame = Frame(app_win)
main_frame.pack(expand=YES, fill=BOTH)

# Label to show your listbox selection
selected_var = StringVar()
Label(main_frame, textvariable=selected_var, font=("times", 12, "bold")).pack(
    expand=YES
)
selected_var.set("No selection")

# Label to show your calculated result
result_var = StringVar()
Label(main_frame, textvariable=result_var, font=("times", 12, "bold")).pack(expand=YES)
result_var.set("No result")

# Container for the listbox & scrollbar (embeded in the main frame)
listbox_frame = Frame(main_frame)
listbox_frame.pack(expand=YES)

mylistbox = Listbox(listbox_frame, height=5, width=10, font=("times", 18))
mylistbox.bind("<<ListboxSelect>>", select)
mylistbox.grid(row=0, column=0)
mylistbox.insert(END, *items_for_listbox)

scroll = Scrollbar(listbox_frame, orient=VERTICAL)  # the allignment of the scrollbar
mylistbox["yscrollcommand"] = scroll.set  # link the list with the scroll
scroll["command"] = mylistbox.yview  # link the scroll with the scroll
scroll.grid(row=0, column=1, sticky=N + S)  # sticky=N+S+E)

# Button to launch a calculation
button = Button(main_frame, text="Fläche der Absorber", command=calculate).pack(
    expand=YES
)

app_win.mainloop()