E29: No inserted text yet
 

I want to input '^@'(caret) through vim.
Whether pressing Ctrl+2 or Ctrl+shift+2,it always
says:E29: No inserted text yet.
But if i want to type other caret notation,eg '^M',simply
pressing Ctrl+M.
Why ^@ cannot input?

press first Ctrl-V and Ctrl-2

I find some control character,like '^L','^M',which could be type directly.But some,eg ^@,^A,must be input using first ctrl-v and then Ctrl+something.
What is the different between them?

If I remember well the standard way is to press Ctrl-V and <special char>
You do not need to use Ctrl-V only for a limited character set, like the letters, numbers and "usual" keys: tab, enter ..... But I can't recall the correct definition, you may need to check the manual.

which manual?

In addititon to Basic ASCII Codes ,range in(0x00~0x7F),how can i
input Extended ASCII Codes(0x80~0xFF) through vim?

https://unix.stackexchange.com/quest...al-codes-in-vi
https://sanctum.geek.nz/arabesque/sp...acters-in-vim/

start vim, enter :help intro, look for Insert Visual mode.
In general I would suggest you to read the help (but need to learn how to use)