This question already has an answer here:
This question is about an error of a sign of
MikTeX in my computer with Debian. This error is
E: El repositorio «http://miktex.org/download/debian stretch InRelease» no está firmado.
This means that:
The repository «http://miktex.org/download/debian stretch InRelease» is not signed
when I write
apt update in the terminal. I don't know if its topic is Tex-LaTeX, Unix & Linux, or Stack Overflow. I'm going to make this question on Stack Overflow, but I think it is off-topic.