It says there is supposed to be banner-looking flag icon to bookmark a page, but I don't have one next to the magnifying glass icon. How can I add a bookmark?

You appear to be using Online Premier.  Do you not see this on your screen as in the example below?

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 FAQ:  Using flags to bookmark a page
https://ttlc.intuit.com/community/using-turbotax/help/can-i-bookmark-or-save-a-certain-spot-in-turbotax-so-i-can-find-it-later/01/25915

I sure don't.  The other 3 icons are there but not that one.

Log out of TurboTax.

Empty your browser Internet cache.

Delete any Intuit-related cookies.

Close the browser and reopen it. 

Then open a private window (aka private browsing, incognito mode, etc.) and go to TurboTax website.

Does that make any difference?

If not, see if another browser makes a difference.