What exactly are you doing? Are you using the excel importer? If so, do you have a domain model that matches with the data in the excel sheet?
Or are you using the App based on spreadhseet functionality in Mendix Studio? If so, what exactly happens?
You might want to start over using this technique to create your domain model: https://docs.mendix.com/studio/start-with-data#excel-type-attribute-type
If not follow these steps to use the Excel import functionality: https://docs.mendix.com/howto/integration/importing-excel-documents#1-introduction