When I try to sign, it says that the .cer or .key file I selected do not correspond to an e.firma issued by the SAT

The .cer file you selected does not belong to an e.firma issued by the SAT

To sign documents with e.firma, Mifiel only supports public certificates (.cer files) of advanced electronic signature issued by the SAT. If you try to use a different certificate such as HTTPS/SSL certificates used on websites, you will not be able to sign your document in Mifiel.

You selected a file with the extension .key that does not belong to an e.firma issued by the SAT

Regarding the private key, we only support advanced electronic signature .key files signed by the SAT. If you try to use a file with a .key extension corresponding to a Keynote presentation, for example, it will not be possible to sign.

Both files can be damaged by improper handling, for example, trying to open them with an inappropriate program.

Make sure you use .cer and .key files that correspond to an e.firma and are not damaged. If your .cer file is damaged, you can recover it from the SAT website. However, if your .key file is damaged and you do not have an intact copy, you will have to request a new e.firma at the SAT offices.