Naruto recovered from a pretty big hole in his shoulder while fighting with Sasuke. Madara was cut in half and could still fly, and Naruto flies, so it would stand to reason that he can recover at least somewhat like Madara. Not to mention how he kind of copied Kakashi's eye, saved Guy-sensei, and tried to save Obito even just before he died.
If Naruto has mastered Senjutsu like Madara and Kaguya, and he has aptitude for recovery, why didn't his arm grow by itself?
Naruto lost his Sage of Six Paths (SOSP) power after his fight with Sasuke. It was mainly due to the fact that he lost the chakras of all the bijuus (tailed beasts) that he had. So, as a result of losing the power of SOSP, he also lost the ability to heal.
Naruto actually 'lost' his SOSP after using a tremendous amount of chakra to seal away kaguya. So after the fight with Sasuke, he must almost have depleted it.
Coming back to the point, while fighting Madara Uchiha and Kaguya, he still had his powers and abilities (levitation, the truth orbs, healing, etc.) It was when he actually sealed Kaguya with the help of Sasuke, did he use up his SOSP powers/chakra. So, he lost the truth orbs, the SOSP cloak, ability to heal, etc.
Now, why did Sasuke not lose his rinnegan? That was because, Indra's chakra is needed to only awaken the rinnegan, and not to maintain it.
So, the reason why Naruto could not heal his own arm was because he lost his powers when he sealed Kaguya, but could use levitation , etc. before sealing Kaguya, that is, while facing her and Madara.
The healing ability dosen't come from the Six Paths Sage Mode, but from the yang seal on Naruto's hand. Once Kaguya was sealed, the mark went away, and so did his ability to heal things. You'll notice that, when healing Guy, a seal looking like the sun mark is left over Guy's eighth chakra gate.
Naruto still has Sage of Six Paths power. Check this post out for more info.
The problem, however, is that in Boruto: Naruto the Movie, even though he can still fly and turn into Ashura Kurama Chakra mode, he doesn't have his truth seeking orbs anymore.
Therefore, it's possible that even though he still has Sage of Six Paths mode, he's lost some of his abilities, maybe even the power to regenerate himself and other people.
My theory is that probably Naruto's arm didn't grow because of the extraordinary pressure created by the attack of Sasuke and Naruto.
There are certainly cases in which arms can be healed by healing jutsu, but what if all the cell and even every single particle in the arm side was destroyed? Then, the resurrecting of the arm would be impossible because no matter what, there is not a single jutsu in the world which can recreate cells.
Although this is just a theory, I believe this is what happened.
As I remember, there were marks, that were given to both of them, which disappeared after they sealed Kaguya. While some say that the marks awakened their abilities, but they might also be explained in another way.
There is no indication that I remember, that Asura got a mark when he got the SOSP power mode, therefore the marks could only be the sealing jutsu. So they both got power upgrade and sealing jutsu powered by the SOSP. Then even after they used it, Naruto still had the same abilities as before, same cloak, he could still levitate, he could levitate all 6 clones of Kurama and still had 3 more orbs (wonder why he didn't make more, they seem to be using them up). On top of all that, he didn't really seem to gather any/much natural energy, until he made those 2 crazy Rasenshuriken.
To be precise, he never really displayed the need for healing like Obito, Madara, and Kaguya did, because he didn't get hurt, until the end fight. He did seem to display slight decrease in power/compensated by alternative power and skill in the following 2 movies, e.g. super punch, ability to separate from Kurama, a beast size Rasengan, still levitating on the moon, healing quite fast from a sword wound in a minute or so in the manga.
So all in all, it is the extreme level of healing and the truth seeking orbs that are really missing later, maybe as existing answers pointed out by not having all the bijuus like Obito, Kaguya and Madara, actually that is when they were making them. Although Obito made a staff even after being exhausted to deflect Madaras orbs.
But then Naruto get Kurama times 2, so I think that is compensated. Still, he should have been able to make orbs after that. Regarding the arm, maybe he didn't have the energy to heal it at the time since Kurama went to sleep, although even later as an Uzumaki he should still display some amazing healing ability like Karin and his mother.
Naruto still has his Sage of Six Path ability as shown by his eyes with vertical and horizontal marks without the coloring around the eyes. This indicates Naruto still has his six path ability in Boruto. He didn't use Create Life to heal his arm probably because of plot holes.
And also if the author actually did it, it would look way over powered having literal jesus in the story. But the fact is that Naruto does still has the six path ability just like how Sasuke still has Tomoes in his Rinnegan.
I think it’s because Madara had both naruto and sasuke’s power while naruto and sasuke only had a part of it each, also he healed kakashi’s eye, guy, and tried to heal photo with the arm that he lost in the fight which could mean that the healing ability was only in the arm that he lost.
