Peppers need lots of light, warmth and fertilizer. I mean 12-16 hours of bright grow light (~36w) or 8–10 hours of direct sun plus weeklyish heavy fertilizer feedings. How much of each is it getting? Also: were those pistachios salted?

You definitely need to fertilize. It might not be the root problem, but any potted plant needs fertilizer because nitrogen is water soluble and washes out when you water it. The leaves look chlorotic, so lack of fertilizer tracks. I recommend a balanced water-soluble fertilizer. Use it in a dilute application any time the plant shows growth or is flowering.

Once that is going, you can further diagnose issues.