Thanks for reaching out. We're here to help.
What version of Lightroom Classic & the operating system are you working on? Could you please share a screenshot of the folder panel where you are trying to order images?
Custom order using drag & drop is not supported if you are in a parent folder, i.e., a folder with subfolders.
You can go to the subfolder to organize images. You can also freely re-organize pictures in a collection.
Let us know how it goes,
Sameer K 😁