FileNotFoundError - the error says it cannot find the file it obviously did find

I am trying to make a simple program to copy all files with a user specified extension into another folder. The code is as follows and the error is below:

#! python3
# Copies all files from folder with user specified extension
import os, shutil

pmatch = []
num = 1
while True:
    print('Enter path ' + str(num) + ' or "end" to finish')
    pt = input()
    if pt.lower() == 'end':
        break
    pmatch.append(pt)
num = num + 1
pth = str()
for i in pmatch:
    pth = pth + str(i) + '/'

truepth = 'C:/' + pth
os.chdir(truepth)

print('Enter folder name to copy:')
folder_name = input()
print('Enter new folder name: ')
new_folder = input()
print('Enter extension to copy:')
ext = input()

orig_pth = truepth + folder_name 
new_pth = truepth + new_folder 

for folder_name, subfolders, filenames in os.walk(orig_pth):
    for filename in filenames:
        if filename.endswith(ext):
            shutil.copy(filename, new_pth)
            print(filename + ' copied')
print('Complete')

Here is the Error:

FileNotFoundError: [Errno 2] No such file or directory: 'alice.txt'

Alice.txt is one of the .txt files in the folder it is supposed to copy, so it clearly found it? What is going on?

It does not find it because if you do not specify a folder Python will only look for the file in the folder where the script is placed, the fix should simply be:

shutil.copy(os.path.join(folder_name, filename))