You can only meditate with Muse offline using accounts that have already been downloaded. When you're offline, you cannot access the Muse database, which means you will not be able to create a new account or access accounts that don't already exist on your device.
If you want to use Muse offline, make sure to first sign in when you're online. The account will then be downloaded to your device. From then on, you will be able to continue using that account on that device, including signing out and back in, when you're offline.
Note that when using Muse offline, recent changes to your account may not be synchronized. Your session history and any other account changes may not be up to date if you've been using other devices. Data from sessions you complete offline will not sync with our database until the next time you go online with that device and launch the Muse app.