Title says it all; There appears to not be ANY 1.21.6 version of the mod, despite there being a 1.21.5 and a 1.21.7 version-
Also, there is an addon for the mod, "Chalk: Colorful Addon" that IS for version 1.21.6, but using either of them causes the game to not even launch, due to CCA needing the original version of Chalk, which does not exist for the version it is.

Sorry, for that. Around the time 1.21.6 was released I was pretty busy IRL, so I never got around to create a release for this specific MC version.

With the frequency Mojang are doing breaking changes now it sadly is unsustainable to keep all my mods updated to each single one. 💀