Hello, I had an iPad, but I sold it and there was a lot of progress on the Apple id account in Clash Royal, how can I transfer my account from Game Center to an android device?

Most likely you can’t. Not using game center anyway. Game center is for iOS devices only.

If you backed the device up somewhere else than maybe, but if you’ve already gotten rid of it there may be nothing you can do.

You can see with the clash Royal support and see if they have any sort of backup that may have worked and may work cross platforms.

This is probably an issue that would have been better sorted before you got rid of the device.