Black Goku is state to refuse to get access to immortality because he is satisfied of his power and that he wants to become more strong. This wiki says, "In addition, since immortality as a consequence of halting age or death, puts people at the absolute peak of their power, it also leaves them unable to grow stronger through training or, in the case of Saiyans, getting stronger via surviving mortal wounds through Saiyan Power. This is implied by Goku Black's reasons for refusing to use the Super Dragon Balls to achieve immortality despite Future Zamasu's request that he do so in the manga." There is no source for this information.

So, is Goku Black incapable of using the Zenkai (the power of the Saiyajin) or is he incapable of progressing anyway ?  In fact, I want you to let me know if an Immortal can progress by training. Hope you will be interrested by this subject ! Don't forget to link me some sources to prove your arguments.