Einen is used when addressing thr tablr to somebody like wir haben einen tisch or du hast einen tisch and ein is used when just defining it like ein tisch or es ist ein tisch (only mentioning table here)
The first thing that comes to mind when reading "table" out of context is "furniture-table", rather than "chart-table", so they probably just didn't think of "eine Tabelle". Still, it is a valid translation, so report it next time.
It means both, and both are accepted. Are you sure there weren't any other errors/typos in your answer? ‘A table’ is the suggested translation, so, barring a bug, it's weird that Duo would correct you on that.