I have a lightroom catalog on my PC with the photos themselves on a NAS. I did an import to Lightroom CC from Classic (after optimizing the library) and it completed but there were no files. I deleted the album and tried again but now it says I cannot migrate again. I tried to migrate the old un-optimized catalog but got the same error.
I heard there was an option in the web app in the Account section to delete your lightroom catalog but cannot find it.
Before you started migration, did the dialog say anything about missing originals?
You can delete all your photos in the cloud on LrWeb. You can log into lightroom.adobe.com, press on the Lr icon on the top left corner and click on Account Info. There, under actions click on Delete Lightroom Library.
You could try deleting/renaming the existing local LRCC catalog. You should find it in users/username/AppData/Local/Adobe/Lightroom CC......delete or rename the Lightroom CC folder, then try the migration again. Though you might still have trouble if the Classic catalog's files are on a NAS.