When you are on the column matching stage, you may encounter this error keeping you from progressing to the Review stage of your import. If there is a required field set up on the Template against which you are uploading data, this must be matched (in Portal 3.0, not Workspaces) to a column on your file in order to proceed to the Review stage of the process. This information is necessary to complete the import process.
This error message will not show up if you are in Workspaces, since you can upload multiple files against the same sheet in a Workspace before submitting your data. Since you can only import one file at a time in a Portal, there must be a corresponding column for every required field on the Data Template.