This error is returned because the report is not in English, and does not contain a column called SKU which Linnworks expects for mapping.
The report language is determined by the marketplace in which your seller account was opened. To change the language in which your reports are generated please do the following:
1. Go to your Account Info page and scroll down to the Feed Processing Report Language section.
2. Click Edit to change the setting.
3. Under Set Default Language for Feed Processing Reports on the page, select your preferred language option (English).
Once you have done that, please go back to Linnworks, open the mapping screen for Amazon, and click Re-request report, so Linnworks would request a new report which would have its column headers in English.