CNN is reporting that current and former members of the Trump Whitehouse fear Trump's wrath if their testimony to Mueller is revealed. These people are unnamed in the CNN report. That makes sense I guess. Assuming that CNN hasn't just made this up- an assumption that I believe is safe given Trump's habit of publicly embarrassing those staff who displease him - this says much about the state of affairs at the current White House.
One essential thing of many for the most powerful man in the world is that he listen to his staff. They aren't there simply to do his bidding without question or consideration. They are there to offer sound and considered advice on the many topics on which they hold expertise. No President can be the most knowledgeable person on every subject. Even if he is the smartest guy in the room he still needs other perspectives in order to ensure that he has the chance to choose the best course of action.
Having a staff that fears his wrath will curtail their freedom to uncomfortable options when it is time to consider all of them. Whether that phenomenon is due to Trumps' bullying or an overpowering ability to select only those who will cower no matter how well they are treated is a HUGE problem.