If your app is supposed to work on an iPad then you need to provide screenshots for the iPad. You can do this by building and running on an iPad Simulator on your Mac.
If your app is not going to support the iPad then you need to load your project in Xcode, and remove the platforms it doesn't support. This is done in the General tab of the project view's targets. You know there this is? In the left-hand list of your prject's files, the very top blue item is your project name. Click that, and the right-hand side shows your project and a list of targets. Click the targets one by one and remove the iPad from there.
You'll need to rebuild the project and confirm it all works, and submit it to App Store Connect. You shouldn't need to add iPad screenshots then.
Topic:
App Store Distribution & Marketing
SubTopic:
App Review