I want to add text highlight feature in pages to toolbar

I should add that once you highlight a passage of text in Pages, and while it remains selected, you can select Character Style and press the + button to add that highlight color as a new character style. Even assign an F1 - F8 function key that would enable a one-click application of that text highlight color to newly selected text.

See Create and use character styles in Pages on Mac - Apple Support

I still believe is the wrong application for this purpose and the above is if you are determined to continue using Pages for this purpose over the PDF tools that I mentioned previously.

Because there aren't. Anything you want to highlight with a background color is done via the Text Background colors — after you select the text you want to highlight. Pages isn't really the tool you need for highlighting text passages in a book.

Instead, export that Book to PDF from Pages and use the text highlighting annotation in Apple's Preview, the current Adobe Acrobat Reader DC, or the Skim PDF reader. The latter is especially designed for the highlighting purpose.