You can't post any data to the databases if you are in demo mode and have exceeded your demo limits. You can find out more about this
here.
That being said if you want to test a feature in RetailEdge or just make sure you are doing everything correctly, an easy way to do this is to switch over to the Demo DB.
To do this choose
Server Connection from the
Tools|Settings menu in RetailEdge. Then change your DB Name to RetailEdgeData_Demo from RetailEdgeData. You will then be working on the Demo Database and wont harm any of your real data. If you want to work on your real data, you can make a backup of your real data in the RetailEdgeData database (or the database that has your real data) and then Restore this into your Demo Database.