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.