@Lraem234 what you are seeing here is a black tip rot, which is common issues for cuttings. But that does not look too good. By now the rot might have travelled down already but the cutting seems too short so not sure if cutting it down would help.
I hope other members would chime in.
If that cutting is mine, I will try to at least save it by cutting the part with the black tip until you see healthy branch with white sap coming out. Cleaning and sanitizing the knife each time helps prevent the spread of the rot.
Let the cut part callous for 2 weeks before trying to root it again in a moist not wet fast draining soil. Hopefully you can put it where the pot is put on a warm pavement, bottom heat is important in rooting plumerias. Hold off watering because there are no roots until you see 4 sets of leaves.
Good luck. Keep us posted if you are able to save it or not.