Translation:Do not be afraid of the brightness of this fire.
Nyala can mean flame, which seems like the most obvious choice here. From KBBI https://kbbi.web.id/nyala
nyala/nya·la/ n 1 massa gas yang berpijar yang timbul dari benda yang terbakar: -- api itu masih terus berlangsung; 2 cahaya yang keluar dari api (sesuatu yang terbakar): dari jauh kelihatan kelap-kelip -- lampu di rumah itu;
1 A mass of incandescent gas that rises from a burning object. "The flame of that fire continues to burn" 2 Light that comes from a fire (something that's burning) "From far away we can see the flickering of the house lights.
I took that definition right out of the official dictionary...
EDIT: Ok, so "nyala api" means flame, and "nyala lampu" means "Light from a certain source"
"Nyala api" is much, much more natural to be translated as "flame" rather than "brightness of the fire". If you want to use the word "nyala" for "brightness", perhaps using "nyala lampu" (brightness of the lamp) would fit better in that sentence.