hi i have problem with minecraft name tag

Hello ! I've buy a minecraft java account from minecraft.net and when I try to change my name tag to Not_Natsuka it said the name is blocked i swear there is no one using it also it doesn't mean swear i just want to use this name Not_Natsuka Natsuka with japnaise mean Summer air so what is the problem here if you can please edit my profile and replace my name tag with Not_Natsuka

Thanks You have a nice day