Symptoms
When trying to do a product import from Excel, an 'External references are currently not supported' error is thrown.
Cause
The Excel worksheet has references to other worksheets or has information formatted in a special way (e.g. 'Currency' instead of Number).
Solution
The entire Excel worksheet should be in plain text with the values for each cell entered into the sheet directly (no references/formulas) and all values should be formatted as text or numbers.