Do not use the ubuntucinnamon-desktop task for cinnamon builds, it doesn't seem to be present in the archive yet. Use the metapackage instead, which seems like a better way forward anyway.

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.809) UNRELEASED; urgency=medium
+
+  * Do not use the ubuntucinnamon-desktop task for cinnamon builds, it doesn't
+    seem to be present in the archive yet. Use the metapackage instead, which
+    seems like a better way forward anyway.
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Mon, 06 Mar 2023 11:27:02 +0100
+
 livecd-rootfs (2.808) lunar; urgency=medium
 
   * Add ubuntucinnamon to the seed-determination switch-case.

live-build/auto/config

@@ -786,7 +786,11 @@ case $PROJECT in
 		;;
 
 	ubuntucinnamon)
-		add_task install minimal standard ubuntucinnamon-desktop
+		add_task install minimal standard
+		# The ubuntucinnamon task is not yet baked into the archive,
+		# but using the metapackage directly should be as good, if not
+		# better.
+		add_package install ubuntucinnamon-desktop
 		LIVE_TASK='ubuntucinnamon-live'
 		;;