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 chapter
As 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.
It depends on which version of Goku you are referring to:
Dragon Ball Heroes
-He used it on Cumber in the one episode and then the next episode he shows up with the Grand Priest being able to do it at will
- The power scaling is really wonky as well
Manga / Anime [Tournament of Power]
He activated the first variant of it (not at will, but through sheer self preservation) when Jiren pushed the Spirit Bomb back at him and it exploded with Goku being sucked into it
As the tournament progressed he kept activating it again and again, but it was the defensive aspect of the form as he was able to block all of Jiren's strikes but not land any of his own.
Near the end of the tournament he achieved the offensive aspect of the form where he basically spawned a galaxy underneath his feet and then proceeded to hit Jiren and negate his energy ball attack after the Vermoud (Hakashin of U11) told him to finish Goku. It is at this point that he achieved the true form of UI.
Then in the final episode where him and Vegeta square off and pay homage to DBZ he says, "I guess I was pushed to the edge in the tournament and did it accidentally" when Vegeta asks him to use it.
In the anime / manga he can't do it at will as that would make him completely over powered and no matter who he fought after would not stand a chance. If he could he would of wrecked Broly with ease no matter how "mad" Broly got in the new movie that recently came out and that would of removed the need for Gogeta.
On a side note. I think the Angels are always in UI mode since their hair is that silver color [but this is just a guess on my part]. The only thing I can say backs this up is that Whis can dodge anything and everything with ease. He is an angel and he has had eons to master UI inside and out.