Perhaps if you describe precisely what you have actually tried, without success, contributors here might be able to provide appropriate help or guidance. The tags appended to your post suggest that you might be using a beta version of iPadOS. If using a beta version, this being early/unfinished software, you should have every expectation of encountering issues.
If you are currently enrolled in a beta programme and wish to return to using the current production version, this support page provides the necessary information to remove the beta from your iPhone:
How to uninstall iOS beta software - Apple Support
Be aware that restoring an earlier version of iOS/iPadOS will completely reset and erase your iPhone. Assuming that you have a pre-existing iCloud or iTunes backup of your iPhone that has not been updated since installing the iOS/iPadOS 26 beta, your locally stored data and files can be restored after returning your iPhone to iOS/iPadOS 18.6.x. if you don't have a pre-existing backup, all locally stored data will be irretrievably lost:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
An iCloud or iTunes backup can only be restored to a device running the same or later version of iOS/iPadOS than created (or last updated) the backup. As such a backup created or updated by iOS/iPadOS 26 cannot be restored to a device running 18.x.
Otherwise, you might be well advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to help with troubleshooting:
Genius Bar Reservation and Apple Support Options - Apple