What are this Pasuk and the following Pasuk coming to teachy us?
Rashi #1 and Targum Yonasan: Based on the following episode hinted here, it belongs to the rebukes that comprise the current Parshah: When Aharon died on Hor ha'Har, and the Ananei Kavod disappeared, Yisrael, threatened by the king of Arad, 1 appointed a leader to return to Egypt, 2 and traveled back eight encampments to B'nei Ya'akan, and from there to Moseirah. At that stage, the B'nei Levi fought with them and brought them back to Gudgod 3 . During the battle, many families on both sides were killed. 4 Whilst in Moseirah however, they attributed the many deaths to the fact that they failed to eulogize Aharon, so they eulogized him there and then. The Torah writes "Sham Meis Aharon", for due to the great Hesped there, it seemed as if Aharon had died there. 5
Rashi #2 (on Pasuk 7, citing Vayikra Rabah): To teach us that the death of Tzadikim in the eyes of Hakadosh-Baruch-Hu is akin to the breaking of the Luchos.
Rashi #3 (on Pasuk 7): To teach us that when they declared "Nitnah Rosh v'Nashuvah Mitzraimah" (after Aharon died), to separate from Hashem, was as severe as the day that they made the Golden Calf.
Rashbam: In answer to the potential query on Moshe - that Aharon died, so his Tefilah for him was ineffective, Moshe is reminding them here that the Golden Calf was at the beginning of the forty years, and he did not die until the fortieth year, 6 after they traveled from Be'eros B'nei Ya'akan, where they had arrived from Moseirah, and they traveled to Hor ha'Har [and he died there]. 7
Seforno: 'Despite the fact that they saw how the Tefilah of a Tzadik (Moshe) shields over the generation - in which case they should have mourned Aharon's death, some of them - or even most of them, whilst pasturing their flocks in the desert, made their way to Moseirah to look for water and grazing-ground for their sheep. Yet when, whilst they were there Aharon died, they did not return to mourn for him or to participate in his burial. Nor did they take the trouble to pay their respects to Elazar, who succeeded him; but traveled straight from there to Gudgodah.'
Da'as Zekenim, Hadar Zekenim: According to the simple meaning, Moseirah and Be'eros Bnei Ya'akan are not Moseros and Bnei Ya'akan mentioned in Bamidbar 33:31. Moseirah is Hor ha'Har, and Aharon died there. Bnei Yisrael left hand Be'eros Bnei Ya'akan to go to Moseirah, and they encountered Hor ha'Har, and Aharon died there.
Targum Yonason: Amalek, who reigned in Arad.
Rashi: Which is synonymous with Chor ha'Gidgad (Bamidbar 33:32), the next stop on their itinerary.
This also answers the question that they traveled from Moseirah to B'nei Ya'akan, and not vice-versa, and the fact that Aharon died in Hor ha'Har, and not in Moserah. See Bamidbar 33:31-33.
This was only six months after Aharon died. Everyone cried over him, and there was war after he died (Bamidbar 20:29, 21:1), Did Moshe need to remind them when he died?! And he did not mention when was Chet ha'Egel, which was before [presumably] most of the current generation was born! Also, Moshe's Tefilah helped to save two of Aharon's sons (refer to 9:20:2:1)! (PF).
In which case, his Tefilah was effective.
Why did Moshe juxtapose this incident to the breaking of the Luchos?
Rashi #1 (on Pasuk 7, citing Vayikra Rabah): To teach us that the death of Tzadikim in the eyes of Hakadosh-Baruch-Hu is akin to the breaking of the Luchos.
Rashi #2 (on Pasuk 7): To teach us that when they declared "Nitnah Rosh ve'Nashuvah Mitzraymah" (where they appointed another for to replace Hashem was compared to the day that they made the Golden Calf.