During Storage Policy Copy creation you will need to select Deduplication to enable Deduplication.
If the Copy was created without using Deduplication, you will not be able to enable Deduplication on the fly, as the Blocks committed to Storage is written uncompressed.
If you would like to use Deduplication, I would recommend creating a new Copy (pointing to a Global or normal DDB) and then Aux Copy all the data from the current Primary Copy (without Deduped). Once the Aux Copy is up to date with the new Dedup Copy, make the Dedupe Copy as Primary.
At this point you can then remove the old undedupe copy and reclaim the space back