Vol. II, Page 86 and 87. Mueller Report
When the President called McGahn a second time to follow up on the order to call the Department of Justice, McGahn recalled that the President was more direct, saying something like, "Call Rod, tell Rod that Mueller has conflicts and can't be the Special Counsel."581 McGahn recalled the President telling him "Mueller has to go" and "Call me back when you do it."582 McGahn understood the President to be saying that the Special Counsel had to be removed by Rosenstein.583 To end the conversation with the President, McGahn left the President with the impression that McGahn would call Rosenstein.584 McGahn recalled that he had already said no to the President's request and he was worn down, so he just wanted to get off the phone.585 McGahn recalled feeling trapped because he did not plan to follow the President's directive but did not know what he would say the next time the President called.586 McGahn decided he had to resign.587 He called his personal lawyer and then called his chief of staff, Annie Donaldson, to inform her of his decision.588 He then drove to the office to pack his belongings and submit his resignation letter.589 Donaldson recalled that McGahn told her the President had called and demanded he contact the Department of Justice and that the President wanted him to do something that McGahn did not want to do.590 McGahn told Donaldson that the President had called at least twice and in one of the calls asked "have you done it?"591 McGahn did not tell Donaldson the specifics of the President's request because he was consciously trying not to involve her in the investigation, but Donaldson inferred that the President's directive was related to the Russia investigation.592 Donaldson prepared to resign along with McGahn.593 That evening, McGahn called both Priebus and Bannon and told them that he intended to resign.594 McGahn recalled that, after speaking with his attorney and given the nature of the President' s request, he decided not to share details of the President's request with other White House staff.595 Priebus recalled that McGahn said that the President had asked him to " do crazy shit," but he thought McGahn did not tell him the specifics of the President' s request because McGahn was trying to protect Priebus from what he did not need to know.596 Priebus and Bannon both urged McGahn not to quit, and McGahn ultimately returned to work that Monday and remained in his position.597 He had not told the President directly that he planned to resign, and when they next saw each other the President did not ask McGahn whether he had followed through with calling Rosenstein.598 Around the same time, Chris Christie recalled a telephone_call with the President in which the President asked what Christie thought about the President firing the Special Counsel.599 Christie advised against doing so because there was no substantive basis for the President to fire the Special Counsel, and because the President would lose support from Republicans in Congress 600 if he did so.
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I've never seen anything like this attempted cover up - truth will out 
... only a few weeks after Mueller was hired as Special Investigator. However McGahn didn't resign then, stayed on staff and testified to Mueller for 30 hours in 2018. For a time Chump's lawyers convinced him to cooperate with the investigation and allow McGahn to talk, in hopes (I guess) that it would keep the focus off Chump.
During all that time, executive privilege was never invoked, and pretty much every staffer cooperated when asked to testify to Mueller. When did executive privilege finally become an issue? When Bill Barr became Attorney General.
