I looked up the web, some said it was calcium, some said overheating or too much sun.
do you guys know what may be the reason?
The leaves are also a little deformed, with puckering; I can't see any yellow mosaic, streaking or barring, so hopefully its not a virus. Otherwise, the most likely explanation is an insect infestation of some sort - I can see what looks like a whitefly on the one of the leaves, but check the underside of the leaves carefully, with a magnifying glass if necessary to try to identify the pest. If you find anything, spraying with neem or insecticidal soap will help. Further info here https://www.gardeningknowhow.com/edible/vegetables/pepper/curling-leaves-on-peppers.htm