I would not call this a problem between Neymar and Mbappe (although formally it is), the main problem is Mbappe, who fell ill with star disease and requires special attitude and additional privileges. PSG bosses themselves are to blame for this situation, since in any top club (and especially in such a money bag as PSG) there will always be more than 1 star, which means you need to select players so that they can work together without conflict.
After new contract extended for Mbappe with PSG looks he has many trouble not only with Neymar but also he has controlling for all team activities until become the reason why Pochettino have leaving his manager position with PSG. Neymar have unlucky position right now when getting problem with Mbappe because he can't get chance with special respond from team management, exactly PSG would priority with Mbappe than Neymar how ever mistake made first by Mbappe. Not easy how controlling with attitude star player like Mbappe and make PSG manager Christophe Galtier have think thousand way how keep make his team solid although faced many trouble days by days.