Hi,

This looks like a case of the blossom end rot disorder. Just like tomatoes, peppers can also be afflicted during their early growth stages and involves calcium uptake by the pepper plant. Peppers require even watering--not too much or too little, enough for soil to stay moist without drying out. Calcium in the soil travels through the watering carrying vessels in the pepper plant and when it is hot (or if we have low humidity) the pepper plants see to its leaves first and doesn't really feed the developing fruit. So you will see signs of calcium deficiency showing up as what we call "blossom end rot" which can show  up as sunken, rotted looking spots on the bottom or sides of your peppers. You'll want to remove the fruits with the rot.

Usually, if the watering issue is satisfactorily taken care of, the problem will resolve.

Here's an information sheet that will help you know what to do for the future:

https://extension.umd.edu/resource/blossom-end-rot-vegetables

Sue