What should I do? Leaves are brown.
See a couple of things going on here @Mocha_Boss-Woodard the brown spots are most likely being caused by one of 3 things.
Your young plant was just introduced to nutrients which often causes an issue called nutrient burn, while this can be problematic it is very common and most plants make full recovery. There is not much you can do at this point if it is Nutrient Burn so I would recommend just letting her tough it out.
Are the leaves that are browning touching the floor of the grow space? In my experience this can also cause the leaves to become damaged. It is not something to be concerned with as the new growth will grow to take its place soon. (I think this is your issue)
Your coco pod is sitting a little low in the reservoir lid. This can cause your pod to become over saturated which can bring on an issue called damping off, where the stem becomes too weak to support the plant causing it to bend/crack/break and ultimately end the life of your plant. I would recommend that you very carefully push the pod up from the bottom until it is flush with the top of the lid. Take great care not to break the pod when adjusting!
Below is a link with some questions that you can answer here on your thread that will give us more insight on your grow and allow us to share more advice and guidance if necessary. Welcome to AG and Grobo Growing, we’re happy to have you!