@DaBlackVenom The S21 series definitely is compatible, where did you purchase the device and is it definitely a uk model? Should be able to update via one of the methods.You could try wiping the cache partition to see of that makes a difference:
1. Turn off the device.
2. Connect To Computer or plug in USB-C Headphones , Press and hold the Volume Up key and then press and hold the Power key
3. When the Android logo displays, release all three keys.
4. An 'Installing system update' message will show for 30 - 60 seconds before the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.
otherwise may need to have the phone checked out.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
