Why Are 3rd Party Audiences Unavailable in Our DV360 Account?

Hello all, our company needs to use specific 3rd party audiences for an upcoming programmatic campaign. However, our agency informed us that our DV360 account does not support 3rd party data segments. According to their message, the DV support team has deemed our account ineligible for these segments. I'm not well-versed in programmatic ad campaigns, but I checked DV360's help documentation and found no eligibility criteria listed for 3rd party data segments. From what I've read, it seems you should be able to search for audiences through a marketplace dropdown when setting up a campaign. Can anyone offer clarity on this issue? Am I missing something on the DV360 help page? Any guidance would be highly appreciated. Thanks!