The checkbox in Service Configuration under WorldCat Discovery and WCL > Local and Enhanced Content > Enable searching of Local Holdings records may be unchecked.
Check that box and the Local Holdings Records you have loaded will become searchable.
If this option is turned on and your barcode is still unsearchable, open the LHR in Record Manager and click save.
This will resync the record and make it searchable.
Refresh your Discovery and attempt the search again.
If your barcodes are still unsearchable, contact OCLC Support with a few example barcodes
There is more available on indexes available for Local Holdings Records.