I hear singers say this all the time: "I just can't riff." Maybe their fear is, if they tried, they wouldn't do it right. Well you gotta start somewhere. Yes, the best riffers out there have been singing riffs for years and they have been building something in their voice called agility. Vocal agility is being able to move quickly from note to the next with accuracy and precision at a faster tempo.
I would start with building overall good foundational vocal technique. Then once you have pretty good vocal technique, start learning shorter span riffs. (like descending major 3rd, very common in contemporary pop/RnB.
More importantly, you have to LISTEN to amazing riffers. The best genre to listen to for highly skilled riffers would be Gospel. Yes R&B has amazing differs too, but the great R&B artists are highly influenced by Gospel singers.
If you've only ever practiced and learned major and minor scales, that’s a good start, but you're going to need to expose your ear to hearing blues and pentatonic scales.
Practice small span riffs slowly and gain accuracy moving from one to the next. Then speed the tempo up and overtime your voice will get use to activating those muscle fibres to be able to riff and run.