Here are some tips if you cannot delete a Facebook page:-

1. Your application must be updated to the latest version.

2. Try using the process on a different device.

3. Make sure you don't have another page recently created on Facebook.

4. Check whether the password entered is correct.

5. The deletion is yet to be fully completed.

Read More:-https://allinsider.net/how-to-delete-a-facebook-page/