Hello Chad, and welcome to planDisney!
I see someone will be traveling to the Black Spire Outpost soon to create something special for themselves at
Disney's Hollywood Studios. I believe that the custom lightsabers are perfect souvenirs that you can cherish and utilize in play when you return home.
Savi's Workshop - Handbuilt Lightsabers is a pretty popular activity, so it is quite possible that the days you are searching could be sold out. I recommend continuing to check for availability as a slot could open at any time prior to your arrival. Also, it might be helpful to search for another date within the dates of your stay because you might find an opening there. You mentioned getting an error message and for that, try accessing the site from a computer using the website, or vice versa from the mobile app. If you continue to have issues, then it might not be a bad idea to call Website Support at (407)939-4357 to get assistance from a cast member who may be able to help.
Please let us know if you have additional questions; we are here to help you plan as you get closer to your arrival date.
Have a magical experience!
Raquel