From the article:
“For months, Mueller told Trump’s lawyers that he needed to hear from the President to determine his intent on key events in the obstruction inquiry. During one tense session in March, Mueller raised the possibility of getting a subpoena to compel the President’s testimony.”
It sounds like Mueller is saying he can’t determine that there is obstruction unless Trump tells him the intent was to obstruct the investigation. Yeah, sure his lawyers aren’t going to let him say in so many words that he intended to obstruct the investigation.