First of all, check for filter updates by clicking the icon with circular arrows in the top right corner of AdGuard's main screen.
Go to Settings → Ad Blocker and make sure that AdGuard Base filter is enabled. Same for language-specific filter(s) for your language. If needed, add them by clicking the Add a filter button.
Go to Settings → Extensions and enable the AdGuard Extra extension.
Open AdGuard Assistant (green shield icon next to the address bar of your browser) and click Block ads on this website to block the ad manually. Then report the missed ad by clicking Report an issue in AdGuard Assistant while on the problematic website.