It’s totally legal to write code like that in Python. However, writing code like
lst = [1, 2, 3, 4, 12]
would be “better” than
lst = [1, 2, 3, 4]
... # code has nothing do to with lst
lst.append(12)
In general, the former one would have better performance than the latter one, but if the latter one is more readable in your case/you have a good reason doing that, then you can ignore the PyCharm prompt.
If it bothers you, you can turn this inspection off in
“PyCharm->settings->editor->inspection->Python->List creation could be…”