Why doesn't "Custom number format" show up in Google Sheets (Google Spreadsheets)?

According to this Google help article, I should be able to do the following to apply custom number formatting in a Google Sheets document.

1. Highlight the data you want to format.
2. Click the Format menu and hover over “Number” (or click the 123 number format icon in the toolbar).
3. Select Custom number format….
4. Search in the menu text box to select a format. You can also add your own custom number format into the text box.
5. Click Apply.

Seems simple enough, but I don’t have a Custom number format when I click on the 123 number format icon, or go to Format->Number.

I do have Custom Decimals, but that only allows you to specify the number of decimals. In my case, I want a custom format that involves the percent symbol, e.g., 0.001234 is rendered as 0.1234%.

I also have More formats and More currencies, but neither of those have Custom number format, nor anything that looks like what I want.