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phrase = input("Enter a phrase")
rev_phrase = reversed(phrase)
i = len(phrase)
while i >= 0:
    if phrase[i] != rev_phrase[i]
        print(phrase + " is not a palindrome")
        exit()
    i -= 1
print(phrase + "is a palindrome")

Trying to reverse the order of a phrase, then check if it spells out the same thing before reversing. However, I am getting an error in line 5 where I am reversing the phrase.

My Error Message:

C:\Users\Mikhail\PycharmProjects\FirstUse\venv\Scripts\python.exe "C:/Users/Mikhail/PycharmProjects/FirstUse/venv/Exersise Six.py"
File "C:/Users/Mikhail/PycharmProjects/FirstUse/venv/Exersise Six.py", line 5
if phrase[i] != rev_phrase[i]
^
SyntaxError: invalid syntax

Process finished with exit code 1

You need to have : at the end of line #5.

EDIT:

You should keep in mind that 'Anna' is considered palindrome as well as 'Step on no pets', 'Was it a car or a cat I saw?'

phrase = input("Enter a phrase")
rev_phrase = reversed(phrase)
i = len(phrase)
while i >= 0:
    if phrase[i] != rev_phrase[i]:
        print(phrase + " is not a palindrome")
        exit()
    i -= 1
print(phrase + "is a palindrome")

Now I am getting this error in #line 5:

C:\Users\Mikhail\PycharmProjects\FirstUse\venv\Scripts\python.exe "C:/Users/Mikhail/PycharmProjects/FirstUse/venv/Exersise Six.py"
Enter a phrasepop
Traceback (most recent call last):
File "C:/Users/Mikhail/PycharmProjects/FirstUse/venv/Exersise Six.py", line 5, in <module>
if phrase[i] != rev_phrase[i]:
IndexError: string index out of range

Process finished with exit code 1

Python uses zero based indexing therefore:

>>> a = 'abc'
>>> len(a)
3
>>> a[3]
/../
IndexError: string index out of range
>>> [a.index(char) for char in a]
[0, 1, 2]

in python indexes are zero-based, thus max index in len(phrase)-1

now, unless you are required (e.g. homework) to use while loop and indexes, there are better , more pythonic ways to complete the task

using slices

if phrase == phrase[::-1]
    print("it's palindrome")
else:
    print("not palindrome") 

or even as oneliner

print("it's palindrome" if phrase == phrase[::-1] else "not palindrome")

if you want to compare char by char

for c1, c2 in zip(phrase, phrase[::-1]):
    if c1 != c2:
        print("not palindrome")
        break
else:
    print('palindrome')

if all(c1 == c2 for c1, c2 in zip(phrase, phrase[::-1])):
    print("palindrome")
else:
    print('not palindrome')