Based from the hint above, do you somehow have a disablewallet=1 entry in your bitcoin.conf file or as an additional start parameter in your shortcut?
Because disabling the wallet will make 'wallet RPC commands' like walletpassphrase disabled.
After I downloaded the Bitcoin core wallet, no other operations have been performed. If I disable it, how can I find out? Then how to unban it? I'm a novice, please!
Check your Bitcoin data directory (
location) if there's a "
bitcoin.conf" file inside it, if there is, open it using a text editor and check if there's a line with
disablewallet=1.
Remove that to enable wallet function.
If you're on Windows and running using a shortcut, check its properties and look for
-disablewallet at the end of the Target, if there is, remove it.
Also, you should answer NotATether's question about the site where you've downloaded Bitcoin Core.