[reason] doesn't exist as [reason] in CB Auto Actions. It'd be whatever the variable is that has $reason for the trigger you're acting on, which onBeforeUserApproval and onAfterUserApproval don't include. It's available in the POST data, but the variable name includes the user id. The below might work, but I'm not sure.
[post_comment[var1_id]]
Rejecting a user however already sends an email so you shouldn't need a auto action for this.