Apple Watch Series 6

They are usually charged to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge.  As long as you are able to charge it to 100% before you set it up (which is recommended) you should be OK. For more info, see the 'Store it half-charged when you store it long term.' section in:

Batteries - Maximizing Performance - Apple

Yes,it could have been in storage a long time,mine didn't have enough power to turn on when I bought it.