What do you mean by not allowed? Confirmable-in-advance RUC space is not very common on domestic D1 flights, particularly on flights like yours where there are only 3 seats left so Delta understandably wants to save those for paying customers. After check in your RUC should put you higher on the upgrade list (although many people try to use RUCs on those flights so you won't necessarily be #1, and some days might not even be in the top 5 depending on the makeup of who's trying to use RUCs on any given flight).

Delta.com will also sometimes show that Global Upgrades can be used to confirm domestic D1, which is a (somewhat confusing) result of the new rules since 2/1 outlined in the wiki -- basically GUCs get access to slightly more inventory for domestic D1 than RUCs, but not last seat availability as with domestic F. But just because a flight may show that domestic D1 is only available using GUCs, you can still waitlist RUCs, which at worst should give you an improved position on the day-of upgrade list, and occasionally may result in the waitlist request clearing before that.