Oracle Standard can be monitored in DPA with a Category 2 license. Oracle Enterprise requires a Catagory 1 license. DPA is also licensed per Oracle instance. If you have a Category 1 License that you can add to the registered instance you should be able to allocate the license in the license allocation page in DPA.
Here is the article on this and if you need more help please open a support case. Activate your licenses - SolarWinds Worldwide, LLC. Help and Support
Many thanks for the clarification.