My brand new Apple Watch shows a red lightning bolt even when it's been on the charger for several hours

You see this?

Apple Watch battery is low.

Maybe the charger or cable is faulty

or if the battery is depleted, it will take some time to charge.

Did you follow the instructions in the support document where you came from?

For your convenient, click here => If your Apple Watch won't charge or it won't turn on - Apple Support

7% is not enough.

It will need at least 10%-20%.