The anime is available for streaming on Crunchyroll and the Roku Channel. Manga and Volume 21
Because Hana Yori Dango (Boys Over Flowers) has several famous adaptations, Episode 21 refers to different pivotal moments depending on the version you are watching.
If you are looking for the manga content, deals with the aftermath of Kaede Domyoji discovering Tsukushi living in her home. Hana Yori Dango Episode 21
How to watch and stream Hana Yori Dango (Boys Over Flowers) - Roku
While on a trip, news breaks that Shizuka (Rui’s first love) is being courted by a French politician and may marry him. Rui is devastated and tells Tsukushi that "it's over". In a moment of vulnerability on the beach, Rui kisses Tsukushi, which is witnessed by Tsukasa (Domyoji). The anime is available for streaming on Crunchyroll
If you mean the individual chapter, it features Tsukasa and Tsukushi getting stuck in an elevator, forcing them to spend uncomfortable alone time together.
In this version, Episode 21 is a heavy-hitter for the "soulmate" storylines and the lead-up to the finale. How to watch and stream Hana Yori Dango
To protect her friends from her mother-in-law's wrath, Tsukushi agrees to stop seeing Tsukasa.