Can someone help a beginner please ?

looks like start of k-deficiency? if soem spots coincide with margins showing the chlorosis, that'll confirm it. Stuff that starts around the edges tends to be K-related. some others have that potential, but doesn't start like this. lockout due to too much of one ore more nutrients would usually cause multiple, confusing symptoms that are less easily diagnosed. Same with most pH lockouts, but pH will lockout Ca before other things if only off by so much could easily lockout Ca and nothing else, but that isn't the symptom seen here. sounds like you've been a bit conservative on fertilization. This is a good way to start. Always easier to add some than trying to dilute what was given. I'd increase the part of fertilzier that has more K in it. if you haven't fertilized much at all start with what the company instructs. If it is AN or fox farm, cut what they say in half, lol. soil is always a trial and error learning experience. if you use the same products, you can learn to pre-empt any needs as you progress. you learn how to supplement as the soil loses its charge of nutes. some things may deplete before others and you'll need a more complete diet as time progresses as well as a higher overall concetration given. Read and react to plant. Take notes of what you did and what resulted... draw up a better plant next grow... rinse and repeat until you have healthy plants seed to hervest in a consistent manner. can never be 100% but think you are underfeeding based on description. This is by far the better way to learn, if that makes you feel better.