Why cant i back up my iPhone with cellular data😢

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 ...

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.