This error is encountered when trying to import a product which has incompatible characters in the SKU. This could be on a single product listing, or any one of the variations of a variation listing.
We often see examples where the SKU appears to be completely normal but this error is still occurring. This typically happens when a SKU has been copied and pasted from one place to another, and has picked up some 'invisible' characters along the way. If your SKU looks normal, we would recommend editing and deleting the SKU, then typing it in manually to ensure there are no unexpected characters appearing that cannot be seen