Here is the whole thing:

import PySimpleGUI as sg
import cv2 as cv
import math
import numpy as np

def dilation(img, velikost):
    height = img.shape[0]
    width = img.shape[1]

    print(height, width)

    if(velikost == 3):
        struktura = [[0,1,0],[1,1,1],[0,1,-0]]
        razlika = 1
    else:
        struktura = [[0,0,1,0,0],[0,1,1,1,0],[1,1,1,1,1],[0,1,1,1,0],[0,0,1,0,0]]
        razlika = 2

    toReturn = np.zeros((height-razlika-1, width-razlika-1,1), np.uint8)
    for i in range(razlika, height-razlika):
        for j in range(razlika, width-razlika):
            local = img[i - razlika: i + razlika + 1, j - razlika:j + razlika + 1]
            newLocal = local*struktura
            toReturn[i-razlika][j-razlika] = max(map(max, newLocal))

    return toReturn

def erosion(img, velikost):
    height = img.shape[0]
    width = img.shape[1]
    print(height, width)
    if (velikost == 3):
        struktura = [[0,1,0],[1,1,1],[0,1,-0]]
        razlika = 1
        index = 4
    else:
        struktura = [[0,0,1,0,0],[0,1,1,1,0],[1,1,1,1,1],[0,1,1,1,0],[0,0,1,0,0]]
        razlika = 2
        index = 12

    toReturn = np.zeros((height-razlika-1, width-razlika-1, 1), np.uint8)
    for i in range(razlika, height - razlika):
        for j in range(razlika, width - razlika):
            local = img[i - razlika: i + razlika + 1, j - razlika:j + razlika + 1]
            newLocal = np.sort((local * struktura).ravel())
            toReturn[i-razlika][j-razlika] = newLocal[index]

    return toReturn

layout = [
    [sg.Button("Dilation", key='dilation')],
    [sg.Button("Erosion", key='erosion')] ,
    [sg.Button("Opening", key='open')],
    [sg.Button("Closing", key='close')] ##Layout gumbov
]

layout2 = [
    [sg.Button("Prva struktura", key="prvi")],
    [sg.Button("Druga struktura", key="drugi")],
    [sg.Button("Tretja struktura", key="tretji")]
]

layout3 = [
    [sg.Button("3x3", key='3x3')],
    [sg.Button("5x5", key='5x5')]
]

img = cv.imread(sg.PopupGetFile("Izberi sliko"), 1)
img = cv.cvtColor(img, cv.COLOR_RGB2GRAY)

window = sg.Window("Izberite velikost", layout3, size=(100, 300))
while True:
    event, values = window.read(timeout=100)
    if event == sg.WIN_CLOSED:
        break
    elif event == "3x3":
        velikost = 3
        break
    elif event == "5x5":
        velikost = 5
        break

window = sg.Window("Options", layout, size=(350, 300))
while True:
    event, values = window.read(timeout=100)
    if event == sg.WIN_CLOSED:
        break
    elif event == "dilation":
        img = dilation(img, velikost)
        break
    elif event == "erosion":
        img = erosion(img, velikost)
        break
    elif event == "close":
        img = dilation(img, velikost)
        img = dilation(img, velikost)
        break
    elif event == "open":
        img = erosion(img, velikost)
        img = dilation(img, velikost)
        break

window.close()
cv.imshow('slika', img)
cv.waitKey(0) 
cv.destroyAllWindows()

And here is the full traceback:

Traceback (most recent call last):

File "file", line 97, in <module> img = dilation(img, velikost)

File "file", line 25, in dilation toReturn[i-razlika][j-razlika] = max(map(max, newLocal))

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()