How to fix this?

Needs more N

Completely unrelated to the question, but I suggest getting yourself some trays and pot risers. It’ll help keep the floor of your tent cleaner and also allow for airflow underneath the pot. Once you start watering to get a little runoff, you don’t want the belly of the pot to be sitting in a puddle of its own pee. Google some “Gro pro NX pot risers” they come in different sizes and they’re about 5 bucks a piece. Trays will help contain the runoff water.

not enough nitrogen or your ph is off and plant cant uptake it.

Same question, same answer.... This looks like textbook N deficiency to me. Yellow lower leaves with a gradient getting more green as you go up the plant. You might have some calcium issues too with the brown spots, but they definitely need more nitrogen.