Cannot upload a CSV to create a Schema

You can import the header row from a CSV or Excel to create a schema. On the Schemas tab click the "Create a Schema" button. Scroll down to the bottom and click the "More" dropdown. Select "Import Fields from Excel/CSV header..." Proceed to paste in the header values separated by commas, click the "Replace Existing Fields" button, then customize the data types.
Note, you can also use this feature to bulk-add new fields to an existing schema.