Moving to the appropriate Forum space.
converted them to DNGs when importing
If you let Lightroom convert at import then you will only have DNG files- nothing else. (No NEFs, no CR2s, etc)
Later I deleted the raw files
Well then I think you deleted the DNGs. And everything has been deleted!
file is associated with another file in the catalogue
And how are you trying to "match up" your images?
Possibly the thumbnail preview remains in the catalog library. If you search for a preview thumbnail you will probably find it is flagged as 'missing'
The "Way around" is to restore your original image files to your computer from a backup and re-link to the catalog-
saying the particular file is associated with another file in the catalogue
This error message appears because the photo has been imported twice (or more than twice) into Lightroom. You need to find the two (or more) imports of the same photo, delete the one you don't want from Lightroom, and then you will be able to re-connect the photo in Lightroom.