Blossom end rot. Either a lack of calcium or the plant is unable to uptake the available calcium in the media.

If unable to take up the calcium, roots may be too wet or too dry. Stick your finger in it to determine which one.

BER (blossom end rot). There are calcium sprays out there, but effectiveness is so-so. Make sure the pot is draining.

Do check the label on your fertilizer. If the soil comes pre-fertilized, in my experience it doesn't last nearly as long as stated on the package. If you have a mature plant, fertilizing it once in its lifetime might not be enough.