What does "Code completions and multi-line edits cannot be active simultaneously." mean?

[stenci]

I got this error in PyCharm: Code completions and multi-line edits cannot be active simultaneously.

What is the difference between suggesting multi-line changes and changes?

Why are they mutually exclusive?

If there is a good reason for them to be mutually exclusive, then this is an UI bug: instead of being two checkboxes they should be two radio buttons.