My first project as a beginner

[lil_e]

Why the error post? I don't see any error and your code works fine for me.

After running your program, paste the clipboard contents into something. This is what shows up after I copied your program to the clipbaord, pasted to VSCode, ran it, and then did a paste here:

Output:
* import pyperclip
* text = pyperclip.paste()
*  
* # Separate lines and add stars.
* lines = text.split('\n')
* for i in range(len(lines)):     # loop through all indexes in the "lines" list
*     lines[i] = '* ' + lines[i]  # add star to each string in "lines" list
* text = '\n'.join(lines)
* pyperclip.copy(text)

My God, it's full of stars!

The code could also be written using str.replace()

import pyperclip

text = "*" + pyperclip.paste().replace("\n", "\n*")
pyperclip.copy(text.rstrip("*"))

Or using an f'string and a comprehension.

import pyperclip

lines = (f'*{line}' for line in pyperclip.paste().split("\n")
pyperclip.copy("\n".join(lines))

And probably many, many more.

Thank you!!