Where is the slideshow button located? I can't find it.

Are you sure you use Impress? The first button on the toolbar shows Draw icon, as if you’ve just created a new Draw document. And of course, cutting all information besides what you feel relevant surely helps those who try to help.

Right click on Standard Toolbar and make icons visible. See attached screenshot…
Or use shortcuts [F5] and [SHIFT]+[F5].
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