While you are typing the text of the transcript in the subtitle editor, the button in the progression panel that enables you to switch over to syncing step is grayed out and cannot be clicked:

You may encounter with a situation when your transcript is finished (that is, you have typed all the text in the video), but Yes, start syncing button is still inactive. This happens if you still have empty subtitle cells. Most often, empty cells appear at the end or at the beginning of the subtitle set; they are displayed in yellow color as on the screenshot below:

You'll need to fill all the empty cells with text or delete them. This will make Yes, start syncing button accessible, and you will be able to go ahead and time your text.