A family of Microsoft word processing software products for creating web, email, and print documents.
AutoText does not exist in Word Online. That's only one of a great many differences between Word in a Windows desktop and Word Online.
If your "chrome laptop" is a ChromeBook, you have no option to download a document to edit it in desktop Word, so you'll have to do without AutoText. A possible workaround is to create a separate document in which you build a two-column table, with a mnemonic in one cell and the expansion text in the other cell on the same row. You can then use the Find function to look up the mnemonic, copy the full text next to it, and paste into the document you're working on. It will be a lot slower than real AutoText, but probably workable.
You can post a suggestion at https://feedbackportal.microsoft.com/feedback to request AutoText, but don't expect it to be added anytime soon.