@mino666666: If you are outside the region that the phone was intended to be used in, then it may not be possible to update your Galaxy S21+ due to software issues that exist in our devices from region to region. As per the suggestion above, Smart Switch may be your best bet if you're unable to update over Wi-Fi via Settings > Software Update > Download and Install. With this in mind, please follow the link below, and click on 'How Do I Get Smart Switch On My PC or Mac' to download the relevant version
https://www.samsung.com/uk/support/mobile-devices/how-do-i-backup-my-smartphone-to-my-pc-or-mac/
Once installed, open Smart Switch, connect your S21+ using the USB cable that came packaged with the device, and look for an update prompt on the landing page, next to your device details.
