why does the ios 15 cannot be updated to my ipad?

If your iPad is capable of being updated to a more recent version of iOS/iPadOS than is currently installed, these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that your iPad may be too far behind the current version to be updated directly from iPad settings. As such, you will likely need to use a computer to perform the update. You will need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

Hi

Sounds like you are excited to download iOS 15!

There are a couple of reasons you may not be able to accomplish this

You iPad may not be compatible with iOS 15 , below are compatible iPads

• iPad Pro 12.9-inch (5th generation)iPad Pro 11-inch (3rd generation)iPad Pro 12.9-inch (4th generation)iPad Pro 11-inch (2nd generation)iPad Pro 12.9-inch (3rd generation)iPad Pro 11-inch (1st generation)iPad Pro 12.9-inch (2nd generation)iPad Pro 12.9-inch (1st generation)iPad Pro 10.5-inchiPad Pro 9.7-inchiPad (9th generation)iPad (8th generation)iPad (7th generation)iPad (6th generation)iPad (5th generation)iPad mini (6th generation)iPad mini (5th generation)iPad mini 4iPad Air (4th generation)iPad Air (3rd generation)iPad Air 2