cuboid = [[i,j,k] for i in range(x+1) for j in range(y+1) for k in range(z+1) if i+j+k != n]
print(cuboid)

But i dont really understand why we made range(x+1) and so on?

don't forget about itertools in python, it's tremendously powerful:

from itertools import product


if __name__ == '__main__':
    x = int(input())
    y = int(input())
    z = int(input())
    n = int(input())
    
    print(
        [
            [i, j, k] 
            for i, j, k in product(range(x + 1), range(y + 1), range(z + 1)) 
            if i + j + k != n
        ]
    )

if name == 'main': x = int(input()) y = int(input()) z = int(input()) n = int(input())

lst = []
for i in range (x+1):
    for j in range (y+1):
        for k in range(z+1):
            if i + j + k != n:
                lst.append([i,j,k])
print(lst)

hii guys i want to know from where i can learn python becoz i have alredy studied python in 12th but i am stilln not able to solve ques like this

Simple Solution ...

x = int(input())

y = int(input())

z = int(input())

n = int(input())

l = []

for i in range(0,x+1): for j in range(0,y+1): for k in range(0,z+1): if(i+j+k)!=n: l.append([i,j,k]) print(l)