Please help! What am I doing wrong? I’m a first time grower

That is looking like iron deficiency. Likely culprit is ph imbalance. But it could also be lock out due to excess of something else. What is your ph level? Problem Is alot of stressers can provoke iron deficiency. So it's not as simple as just adding some. Addressing the root cause is the only real course of action. First and foremost check to see if your ph is in range. If that is in range... than fathom the lockout idea. Check tds of medium? Is it high? Than u can flush to lower tds ppm of medium. Allow plant to recover and properly absorb minerals. If tds isn't high... than start looking into other environmental stresses that could bring on the deficiency... but I'm sure the buck will stop at ph or tds. Good luck fsm