If you are part of a teams it is indeed not possible at the moment to download assets if you have a "viewer" account (there are licences in your org but you have not been attributed one). To go around this and still download the free asseets you can login with a personnal adobe account.
You can create one by logging out of all your adobe accounts and then going to adobe.com and click on "log in" > "create an account" (you can use the same email as your pro one). Then when you log in on Assets with your personnal profile: you should have the possibility to download the free 3D Assets