You're reading your paths from a file with a path per line, but you didn't strip the newlines from each string, so you're looking for files that include a newline character, \n. As your error message notes, the invalid path was:

'C:\\Test\\Project1\n' <-- Single slash followed by n; directories are double slash separated

Strip them off as you read, and you'll be fine:

for line in infile:
    line = line.rstrip("\r\n")  # Removes all trailing carriage returns and newlines
    if line[0] == "R":

I guess you should use Path.iterdir().

for pth in dir_.iterdir():

    #Do your stuff here

I think you want:

for a in source_path.glob("*/*/*"):
    if a.is_dir():
        print(a)

From the documentation: The ** pattern means “this directory and all subdirectories, recursively”, whereas one * is a text wildcard.