Howdy, John! Thanks for stopping by planDisney and asking your question!
Your trip to the Disneyland Resort is in a few days! How exciting! It sounds like you have already made your theme park reservation for your one-day ticket which is why you are getting that error. If you
purchased your ticket on the Disneyland Resort website, you selected a date and which park you wanted to visit before checking out. That was your
theme park reservation.
If you want to visit both Disneyland Park and Disney California Adventure Park in the same day, you will need a Park Hopper ticket. If that's what you purchased, then you made a reservation for the park you want to start at that day and that's the only reservation you need to make. At 1:00 p.m., you can hop over to the other park and continue hopping back and forth the rest of the day.
If you purchased a one park per day ticket, you will need to upgrade your ticket to a Park Hopper at a Disneyland Resort ticket booth in order to visit both parks on the same day.
I hope this information is helpful. Have a fantastic time at the Happiest Place on Earth!
-Linda