Google coding interviews happen in a Google Doc or a similar plain-text editor. No autocomplete. No syntax highlighting. No ability to run your code.
This means you need to write correct code from memory and trace through it mentally. Practice writing code on paper or in a plain text editor. If you've only ever used an IDE with autocomplete, this will feel uncomfortable at first.
Your interviewer watches you type in real-time. Syntax errors and typos signal lack of fluency with the language.