Can Goku activate Mastered Ultra Instinct at will? So Goku trained with the Grand Priest and he shows he can active Ultra Instinct Omen at will. But now he's being beaten and he didnt activate Mastered Ultra Instinct. Before, he showed he could activate Mastered Ultra Instinct when stressed. Can Goku activate Mastered Ultra Instinct at will? Does the manga show something about it?
As far as Dragon Ball Super goes, he clearly mentions it at the end of  Super to Vegeta, after the T.O.P, that he cannot tap into UI and also tells Vegeta that he hasn't been able to tap into it after the T.O.P, in the new manga chapterAs far as Dragon Ball Heroes is concerned, we don't know whether Goku trained with the Grand Priest. We only know that the Grand Priest gave Goku a new set of clothes and Healed him.He most likely figured out how to tap into the Defensive part of UI on his own as he utilizes it to save Trunks.Hearts comments on the fact that Goku hasn't still mastered the form. Whis clearly stated that the attack version of Ultra Instinct is significantly harder to master compared to the defensive portion. If Goku did have the attack version, he would've used it against KamiOren or to destroy the glass as Goku attacks are significantly more powerful in this state.Hence, I think it is safe to say, Goku is yet to master the ability to tap into the Completed Ultra Instinct form whenever he wants to.