Passwordless Login

David is right that generally this is a bad idea. There are occasionally good reasons for doing it, or doing something similar (e.g. automatically logging into serial consoles for lights-out management), but you haven't provided any indication as to why it makes sense for you to do it this way.

Caveats aside, the invalid command name is not coming from the spawn line but from the [sudo] in the expect line. Expect is based on tcl, which treats [] square parentheses as special characters indicating command substitution. Additionally, the value passed to expect is a glob pattern not a fixed string, and [] square parentheses are also special characters in globs. So the answer you are looking for is to quote those characters twice:

    expect "\\\[sudo\\\] password for chronicles:"

Also note that after sending the password you should probably include another expect line to wait for the root shell prompt.

The secure way to access a server without prompting for a password is through keyed logins over SSH. Don't ever give your password in plain text.

If you simply Google, you will find many articles explaining how to do this. SSH login without password is a perfectly fine explanation.

[] is interpreted as "command quotes" ("command" as in "Tool Command Language", which is what Tcl is short for) in Tcl.

{} is the strongest quote in Tcl, you can use it to prevent any interpretation:

expect {[sudo] password for chronicles:}

of course you could also just omit [sudo]:

expect "password for chronicles:"