how to use input function in multiple if statement

Your level_2 is only defined if level_1 is 'left'. If it's not left then level_2 is not defined at all which throws the error.

Imagine if my level_1 is not "left", then when I get to the line which says level_2.lower() then python doesn't know what the value of level_2 is because it's never defined so it can't run the .lower() function

What you have to do is assign an initial value to level_2 before the if checks begin so that you dont get that error

level_2 = ""

level_1 = input("there is two doors infront of you type 'right' for the right door and 'left' for the left door.\n" )
level_1.lower()
if level_1 == "right" :
  print("this door is closed your level is too low to open it.\n")
if level_1 == "left" :
 level_2 = input("you found a two ladder one leads to upstairs type 'upstairs' and another leads to the basement type 'basement'.\n ")
level_2.lower()

The problem is that level_2 is only being given a value if you choose left in level_1. If you choose right, it prints this door is closed your level is too low to open it. but then level_2 stays undefined so it raises the error. You could do the above answer, or you could add a while loop that runs until level_1 is left.

print("Welcome to the dungeon.")
print("Your mission is to find the ursine sword.")

level_1 = ''
while level_1 != 'left':
 level_1 = input("there is two doors infront of you type 'right' for the right door and 'left' for the left door.\n")
 level_1.lower()
 if level_1 == 'right':
    print("this door is closed your level is too low to open it.\n")
 else:
     pass

level_2 = input("you found a two ladder one leads to upstairs type 'upstairs' and another leads to the basement type 'basement'.\n ")
level_2.lower()
if level_2 == "basement" :
    print("hisssssss ,  you got bitten by snake game over")
if level_2 == "upstairs" :
    level_3 =int(input("it seems like the final room thre is three swords infront of you two of them must be traps chose '1','2' or '3' "))
if level_3 == 1 :
    print("you released a powerful ghost you'r dead")
elif level_3 == 3 :
    print("you opened a portal to a cater you're dead")
elif level_3 == 2 :
    print("congratulations you found the ursine sword")
else :
    print("game over.")

You get an error because level_2 only gets defined when level_1 is "left". If level_1 is not "left", your code will continue to throw errors.

What you can do is run lower_2.upper() in the second if statement. That way you only call the upper method on lower_2 after its defined. For example,

if level_1 == "left" :
     level_2 = input("you found a two ladder one leads to upstairs type 'upstairs' and another leads to the basement type 'basement'.\n ")
     level_2.lower()