Try booting into Internet Recovery Mode using Command + Option + R. Unfortunately some Macs tend to default to the oldest version of macOS compatible with the Mac.
Do you have access to another 2014+ Mac so you can create a macOS 11.1 Big Sur USB installer?
https://support.apple.com/en-us/HT201372
Once you boot the installer make sure to erase the whole physical SSD before installing macOS. Even if you did this before I recommend you do it again. This Apple article shows how to reveal the physical drive in Disk Utility:
https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
If you cannot boot the Big Sur online installer and you don't have access to another Mac compatible with Big Sur, then you will need to first reinstall El Capitan and attempt the upgrade to Big Sur again. I highly recommend creating a Big Sur USB installer this time.