The photo is not high resolution, so it's hard to say for sure, but it looks like a mix of a lack of P or K, and overwatering (because of the leaves curling downwards laterally).
My advice: let the substrate "dry" until you stick your index finger in and don't feel it wet, just slightly damp (with the same moisture as when you took it out of the manufacturer's bag).
From there, alternate waterings with flowering fertilizers (one watering with fertilizer, one watering just with water, and so on alternately).
And always measure the pH of all waterings!
Good smokes!
Can you give us more details about the grow to help you better?