I found the problem. Global firmware is divided into: European, non-European and Great British. I had a global large British one. With this firmware, you cannot update after c15. Most likely, when the ui 4.0 update will be released and the British version will also be updated. When I used adb my phone got bricked. which how I restored it. I also could not use twrp due to a bug unknown to me. The patches that are on the first page when updated through updates in the phone in the settings menu did not work for me because they are global European and I have a global British one. there was a conflict.
i used this tutorial
https://telegra.ph/Realme-GT-5G-UnbrickFlash-StockChange-Region-03-02 to trick global UK to global Europe. And I immediately received a message about the new c21 update.
post scriptum: press *#899# software version. if you see the operator: GB, then you have the global British version.
Thanks to everyone who answered my questions and read my messages