Ultimaker Cura's marketplace can be reached from the top right corner of the screen. There are two reasons why the marketplace cannot be reached.
Major Cura upgrade
When installing a major Cura upgrade, for example from version 4.x to 5.x, it may be required for plugins to be verified and updated. Depending on the (external) developer, this might take some time. We encourage developers to update their plugins timely, but Ultimaker does not own the plugins. Please be patient as the developers update their plugins, they will automatically appear in the marketplace when updated.
The content in the marketplace is loaded from the internet. Please check your internet connection and ensure all necessary internet ports and web addresses for Ultimaker Cura are available.