series
You don't have enough iCloud space to do a backup. iCloud gives you enough temporary storage to back up your device. You can also transfer all your data free of charge (iOS15 required).
If you do not wish to have your carrier authorize your purchase, you will need to contact the carrier to activate your iPhone and insert your SIM card. This will allow you to use cellular services through your iPhone.
Depending upon your location, you may be eligible to have your order delivered in-store, curbside, or express at the Apple Store.