The monk player in our campaign is trying to go way of mercy but can't seem to access it, despite me owning and sharing Tasha's Cauldron of Everything. Our sorcerer can access the spells from it, though. Any suggestions?

Monk Player needs to check the following:

Check entitlements are in sync. Account > Licenses. Link: https://www.dndbeyond.com/account/licenses

They have 2014 Core Rules, Legacy and Expanded Rules toggles enabled in the Home tab of the character builder.

The character is being made in the campaign where content sharing was enabled by using the Join link.

They are using the 2014 version of the Monk class. When selecting a class in the drop-down Monk appears twice: once under "Core Rules" which is the 2024 version and again under "2014 Core Rules" which is the 2014 one. To use Way of Mercy subclass from Tasha's book they must be using the 2014 version of Monk.

The character must be 3rd level to select the subclass.