Hi Tish,
I know there was a discussion on here sometime ago about a tropical plant that has similar type flowers and not finding seeds. We concluded it possibly being two things and I think this would hold true of your plant.
One, the most probable cause of no seeds is the flowers are thin and tubular indicative of a specialized pollinator. So if it is some tropical bug or humming bird, you are not going to get fertilization. You can try hand pollination with a small paintbrush sticking it in all the flowers. If this produces no seed, then it might be possibility number 2.
This plant might need another plant for pollination. Some plants require a second plant to have successful pollination.
There is a third possibility when plants don't make seeds is that they are sterile. Being that the plant is not a named cultivar, I don't believe this is the case.
So give the paintbrush trick a try and see what happens.
Remy

Hi Remy! Thank you so much for responding ! I'm sorry for the delay in my reponse, but I've been out sick. My sister-in-law likes the challenge and satisfaction of propagating, but this Manettia luteorubra will have to go unachieved :) Not worth it when the plant was only $5. Maybe she'll like some seeds from some of my other plants, she has plenty to choose from...

Tish