If the Design Team is trying to Upload a Design and nothing is appearing in the Inverter drop-down, an admin can check this:
In the bottom-left corner, click Settings
Click Designs > Design Teams > select the Design Team being used
Scroll down to Authorized Orgs
Ensure the org the design request came from is listed under Authorized Orgs, if not, add & save.
Additionally, check the inverter pricing option to make sure there are no restrictions set by targeting on markets, authorized dealers, or any other circumstances that would block the inverter from being used.