Shopify rejected the file without telling you why. Here is how to find out.

When Shopify says 'file could not be imported' without details, work through these six checks from encoding to file size.

The six causes, in order of frequency

Work through these from cheapest to most expensive to fix:

Encoding: the file is ANSI/GBK/Shift-JIS instead of UTF-8 - Shopify cannot parse it.
File size: above Shopify's limits (15 MB for product CSV uploads).
Row count: very large files time out before import completes.
CSV syntax: unescaped quotes or stray commas break the parser at the start.
Headers: unrecognized header row can fail validation early.
Upload interruption: the browser or network dropped the request.

ShopTools checks encoding, size, CSV syntax and headers in one pass and tells you which of the six causes your file hits - instead of guessing.

Re-save the file as UTF-8 and retry - the fastest fix and the most common cause.
Check the file size; compress or split anything over 15 MB.
Open the file in a text editor and confirm quotes and commas are valid CSV.
Compare the header row against the Shopify template.
Try importing a small 10-row sample to isolate the problem.
Retry from a stable connection if the upload drops.

Before: Error: File could not be imported (no details) After: Root cause found: ANSI encoding, re-saved as UTF-8
Before: Error: File could not be imported (20 MB file) After: Split into 2 files under 15 MB, both imported

Why does Shopify give no error details?: The file fails before it is parsed - encoding or size problems leave no row data to report, so you get a generic message.
What is the maximum CSV size?: Shopify's product CSV upload limit is 15 MB. Files above that should be split.
Can a single bad row cause this?: Unlikely for this error - syntax or encoding problems are file-wide. Row-level errors usually show a row number.
Does ShopTools fix the file for me?: It identifies which of the six causes applies and re-exports a clean UTF-8 file - the fix for the most common causes.