The error is telling you that an argument is missing to your request. You need to specify what information you want to get. See this documentation to understand how to request data using OAI: Open Archives Initiative - Protocol for Metadata Harvesting - v.2.0
An example would be to request all sets in your install, it would look like this: http://agrisost.reduc.edu.cu/index.php/index/oai?verb=ListSets If you go to that page, you can click on some links to access more data, and then you can also understand how to build OAI requests for OJS.
Regards,
Bruno