Error 8541 means the value you submitted for a field differs from the value in Amazon's catalog for that product. Your file states one value and the catalog states another, and Amazon keeps the value its catalog already holds.
The processing report gives five entries for each failed row: the record number, the SKU, the error code, the type, and a message. Record 1 is row 3 of your file, so the row for a record sits at the record number plus two. For error 8541, the report shows both values, the value from your file and the value in the catalog, so the difference is visible before anything is decided. The report does not say which of the two values is right for the product. The catalog value can be older than yours, and it can describe a different variant of the product.
A product page on Amazon is built from the catalog record for the product. When your file sends a value for a field that the catalog already holds, Amazon compares the two. Your row is accepted for that field when the values agree. While they disagree, the catalog value stands and your row returns error 8541.
Where the catalog value is the wrong one, editing your file to match it would put a wrong value on your listing. The catalog record is the thing to change in that case. The record is corrected through Seller Central, by editing the listing there or by opening a case with Seller Support for the catalog value, and the value in your file is accepted once the record and the file agree.
A submission can still carry other errors beside 8541, including errors the file cannot fix. A missing brand registration or approval returns error 5665. A missing permission to list in the category returns error 8026. The diagnosis lists those errors separately, with the reason and the step to take in Seller Central.
The free diagnosis reads your failed file and the processing report Amazon returned. It shows every error by SKU, states which errors we fix automatically, which ones need your answer, and which ones the file cannot fix. For 8541 it shows the change toward Amazon's value and waits for your confirmation before anything is applied. We never overwrite your values without your confirmation. You can start from the FixFlatFile home page.
Requirements differ by category, and Amazon decides whether a file is accepted.