There's nothing to log in to on an iPad. Not sure what you did. If you have Screen Time restrictions in place it would disable such options as deleting the history and website data from Safari. You need to go into Screen Time and turn off the restrictions, so you can access the "Clear History and Website Data" option.
For instance, if you have set Website restrictions in Screen Time ➜ Content & Privacy Restrictions ➜ Content Restrictions ➜ Web Content set to anything other than Unrestricted, the option to Clear History and Website Data, will be grayed out.
You need to identify what you have turned on in Screen Time that is preventing you from doing this. Only you know what options you have enabled there.