Had the same problem. The error message is misleading.
Change your declaration from
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FAbilityActivateDelegate, UDEPRECATED_Ability*, Ability);
to
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FAbilityActivateDelegate, UDEPRECATED_Ability*, Ability_DEPRECATED)
and that should clear the error.