Can the serial number for a one-time purchase be used on iPad or iPhone?

No, it cannot.

The serial number for the Clip Studio Paint one-time purchase is available only on Windows/macOS devices.

In order to use the app on iPad or iPhone devices, please download the app from the App Store and sign up for a monthly usage plan.

App Store (iPad)

App Store (iPhone)

For more information about the differences between a monthly usage plan and one-time purchase license and the supported environments, please click here.