Unresolved reference to asset id
Something in the composition points at an asset that is not here. Frequently the package is fine and incomplete by design.
The message you saw
Unresolved reference to asset id ...Raised by libdcp as UnresolvedRefError and surfaced by dcpomatic2_verify (DCP-o-matic 2.16.91). Our browser check reports: CPL ... references MainPicture ... which is not in the packing list.
Read this before assuming the package is broken
Version files are the most common innocent explanation. A VF carries only what differs from an original version, commonly a subtitle track or a replacement reel, and references the OV for everything else. Verify a VF alone and every OV asset it points to comes back unresolved. That is the format working as designed, not a fault.
If you were sent a VF, you also need the OV, and both must be present for the composition to resolve. A festival receiving a VF alone has received half a delivery, which is worth catching at submission rather than at ingest.
The genuinely broken cases
Assets removed after packaging produce this too. Deleting a reel to save space, or pruning what looked like a duplicate, leaves the composition still referencing it by UUID.
Merging packages is the other one. Two DCPs copied into a single folder give you an ASSETMAP describing one of them and compositions belonging to both. Half the references resolve, which makes the result look repairable when it is not.
Sorting out which case you have
Open the composition playlist and look for a MainPicture or MainSound element whose Id matches the unresolved UUID in the error. If the composition also carries supplemental markings, you are looking at a VF and you need the matching OV.
Otherwise, treat the package as incomplete. As with a missing packing list, the fix is a fresh copy from source, because a composition that references assets nobody has is not repairable by editing the files that remain.