====== (<class>) not found in the list of active addons ======

(<class>) not found in the list of active addons error in RPT, what causes it?

The reason is that you do not list the new cfgVehicles or cfgWeapon class in units[] and weapons[] in the cfgPatches section in their configs.

This is very important to do for save games, the mission editor saving the correct requiredAddons into the mission.sqm and activateAddons scripting command.