Okay, Twenty Seventeen makes somewhat more extensive use of the Customizer’s Active Callbacks feature than other themes generally do.
Specifically, the Theme Options tab will not appear unless there is actually an option in it which you can adjust and have it take effect on whatever you’re looking at right now.
See, that view of the site on the right hand side of the screen is a real view. You can click through to things like Pages and Posts and all sorts of other stuff that you would normally see on your site. When you do so and navigate through your site, then different controls appear to let you adjust things that you can see right now. It makes no sense to give you a control that doesn’t seem to do anything visible, essentially.
So, what you see available in the left hand sidebar specifically depends on what you’re looking at in the right hand sidebar.
For the “Theme Options” area in particular, it contains two sections:
1. A control called “Page Layout”.
2. A set of controls called “Front Page Section XX Content” where XX is a number referring to a section on the page.
The first control only shows up when you are actually looking at a Page, or when you’re looking at an Archive Page which also does not have a sidebar enabled for it.
The second control only shows up when you are both looking at the Front Page of the site and when the Static Front Page option is turned on.
If neither control fits with what you’re currently looking at, then the entire “Theme Options” tab will disappear, because it contains nothing relevant to the view you currently have of the site.
So, make sure you’re doing the above, as before, but also check and see what it is that you’re actually looking at. If you’re looking at the Blog Posts or something that isn’t an archive or a normal Page, then you might not see the Theme Options section. That’s normal.