When importing from excel, you must first export a blank template. You can do this by navigating to the empty States table in the ST-Sim user interface, right clicking on the table and selecting Export All. Note that some field that were hidden in the user interface will also be hidden in excel. You can unhide either before the export (by right clicking on the table), or in excel. Paste the data that you want to import into this file, then save it and import from this file.
Alternatively, you can export the template as a csv file and likewise edit it and import it back.
Both approaches ensure, that you have all of the required fields and features in the file you are trying to import.