"Available with Global Upgrade Certificate" means you can use a GUC to confirm the upgrade on that flight now, but you can't use an RUC to confirm right now (the language could perhaps be clearer).

But you can still use an RUC to request an upgrade on any domestic flight. Unless Delta.com says "Available with Upgrade Certificate", you won't be able to confirm the upgrade -- you'll just be waitlisted. You could possibly clear any time before the flight if Delta makes upgrade space available, or if not, you'll have higher priority on the airport upgrade list than if you hadn't used the RUC. On JFK-LAX, the D1 cabin fills up and many people request RUCs, so unless you travel at a very off-peak time, it's often a longshot to get upgraded unless you waitlist an RUC.

You also see this on regular first class routes, like try any JFK-LAS route (because that's a route where RUC space doesn't open up very often) in the near future ... on domestic first class, GUCs now are good for any first class seat, so every flight should say "Available with GUC" (unless first class is completely sold out). But you could still waitlist any of those with an RUC. On the other hand if you try a route like LAX-SFO decently far in advance, it will likely say "Available with Upgrade Certificate" on many flights, which means you could confirm the upgrade now with an RUC.