What mistake did I do

Question

https://code.sololearn.com/cAqTq8c6NdBV/?ref=app

Answer 1

You never made a variable named 'length'. You need to assign the value taken as input to a variable to be able to use them later.
length = int(input("length: "))
will assign the value entered by the user for length in the variable 'length', which you can use later in the program. Do the same for breadth and height to make it work.

Answer 2

You not store the value to variables.
First store the values, then do calculation.
In last line:
Put ,(comma) between String and variable.