Why cant i back up my iPhone with cellular data😢

For information on backing up an iPhone using cellular data, see the following from Back up iPhone - Apple Support

Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.

The iPhone 6 shown in your original post is not one of the "models that support 5G" as shown in my reply. See that part of my reply again below:

On models that support 5G, your carrier may give you the option to back up iPhone using your cellular ...