what does "your device must be synced" mean?

[jimski]

When synchronizing has finished, ActiveSync will say "Synchronized" and the little swirly thing will stop spinning.

Try disconnecting and reconnecting and see if that will get it to sync and complete. Also, don't be fumbling with anything in Outlook or on your phone when syncing or it will just keeping trying to sync what you just changed.

Maybe one of your conduits are confused (mail, tasks, contacts, favorites). If you see ActiveSync stuck on one of them, you can disconnect your device and open ActiveSync on your phone, select options and deselect one or more of the conduits. If you plan to flash anyway, it does not matter much what is synced, as long as that part of the process has completed.

I wouldn't start any flashing until you have gotten past a completed/successful sync.