What is the date of the database version in Sentinel, and on the scanner itself?
Also, when did you try to do this? Today? Or some other time in this last week (or even earlier)?
See the comments in this thread, started by someone else in your state:
Sentinel: - My town just went P25
For your county, Madison, the site on the statewide P25 system was entered into the online RR database last Monday (10/23). But that was
after the database update for that week. (Uniden gathers the changes noted on Sundays, and the database updates for Sentinel & the scanners are scheduled to be released the following Monday). So, if you were trying to do this any time earlier than mid-morning today, you still have the old database version, and the Madison site was not there. That's similar to the issue in the thread I noted above. The OP in that thread was trying to receive Clay County, also on the new system. But the site for that county was not entered into the online database until last Monday, so it also was not present in the Sentinel (& scanner) database until today's update. It is now present in the database dated 10/29/2023.
The Lake County site (Madison) was not in last week's Sentinel database, but it is in the update released today. Since you are a Premium Subscriber, you can import the corrected details via import is you have Butel's ARC Patrol. ProScan does not support the HP-1/HP-2 series scanners. But if you
do have ProScan, as well as the Sentinel version for the x36HP & SDS series scanners, you can import the system into the version for the newer scanners, then export it into an *.hpe file that can then be imported into Sentinel for the HP-1/-2.
You can, of course, append the system into a Favorites list in the HP-1/-2 version of Sentinel. But if you had already done that, you cannot. directly, append a site into a system already in place. What you can do, though, is delete the system & then append the corrected system. You can also, temporarily, rename the P25 Phase II system to something other than the same name as in the database. Then, you can append the Phase II system so that you can get the site details to use in the system already created.