My 2014 ATS had a series of issues with this in its first 500 miles nearly leading it to be a buyback. The underlying problem was a cracked storage canister and the dealer kept replacing different parts before finally replacing the canister and all lines.
Evaporative emissions systems are pretty simple but access to some parts of it is a pain. I would take this to an independent garage instead of a Cadillac dealer which will be much cheaper. A P0451 can be set because of a faulty sensor, solenoid, cracked canister, or damage to the vacuum line that pulls a vacuum on the canister. So if you decide to repair this yourself, start with a careful inspection of the hoses and canister (pull the purge hose and check for charcoal debris if a cracked canister is suspected) after check the canister outer shell.
California and some other states require a longer warranty on the emissions system than most of the country so check the warranty info for your state. I doubt that it is still in warranty (standard federal emissions warranty is gone) but this is part of the emissions system so specific states may have a long enough required warranty. This issue won't cause any issues with drivability so it should be addressed but it doesn't have to be done by tomorrow.