My guess is you're using the phone outside of the region is was meant for. Please provide your CSC code, country of purchase, where you are now, firmware/UI currently installed.
You could try forcing an update via Smartswitch whilst connected to a pc/laptop.
Open settings, scroll down all the way, you'll see "software update", click on it, then press "download and install", see if that helps download and install the latest Android and One UI versions. It could also be that the country you're in is diffrent to the country the phone was made for. Hope that helps.