We can't determine the exact cause of the damage from the photo, but one possibility may be Blossom End Rot. More information would be needed to help narrow-down likely causes, such as what the growing conditions were like and how they were cared for. (For example: were they grown in full sun or part shade? in the ground, in a raised bed, or in containers? fertilized periodically, only once, or not at all, and with what type of fertilizer? what are the soil conditions, and was a laboratory soil test done taken before they were planted? were they near a heat-reflective wall or pavement?)

You can explore our Growing Peppers in a Home Garden web page for links to common pepper problems (pests, diseases, and environmental stressors) to see if anything seems to match (like Blossom End Rot). There is no curative treatment once these kinds of symptoms appear, but individual fruits can be cut off once this type of damage is seen so other fruits can use the nutrients and moisture that would have gone into the ripening of damaged fruits. A range of factors can lead to poor fruit formation, including poor pollination (were the plants covered in a way that blocked bee access? are there other flowering plants nearby that help to attract and feed the bees?), heat stress (heat stress can cause a pause in flowering or the death of pollen for tomatoes, which are related to peppers), and moisture stress (too much or too little root moisture). If too much nitrogen fertilizer was applied, for example, that can cause plants to grow more foliage at the expense of flowers and fruit.

For now, once the plants are done producing or are killed by frost, remove all crop debris so it doesn't harbor any pests or diseases for next year. The end of the growing season is also a good time to submit a soil sample to a lab (they are probably less busy in autumn and winter) so you have information ahead of next year's crops. We can help to interpret soil test results if needed.

Miri