Your text box must be on an individual line of text. It cannot be on a line of text with other text.
Here's where the trick is. You actually don't insert a "text box" per se, but you'll insert a table.
- Go to Insert.
- Select Table.
- Choose a 1x1 table (the top-left selection).
Now that you've created your text box, you'll likely need to adjust it some. To adjust your new text box (table) in Google Docs:
- Move your cursor to the left or right edge of the text box until the cursor changes to indicate you can drag it.
- Drag the edges of the text box to where you want them.
The text will automatically return to a new line if needed when you adjust your text box.