Since you are in flower mode .... it is a potassium deficiency... typical symptoms.
I don’t know which Nutes you are using , but you should be on a bloom mix.
Leaves are a bit pale too ( nitro ) so you can do a couple of things.
1. Mix up a 50/50 mix of veg AND bloom nute for a feed ....
2. Or top soil with new bagged soil then water in ..... it will act like a tea / soil drench.
3. Or Mix up a slightly stronger bloom nute feed then feed.
Ph everything prior to application....
6.3 to 6.5 for soil
5.5 to 6.0 for coco / Hempy / hydro