Just now I downloaded the latest Linux kernel from the official site. With it I downloaded the .sign file. I found instructions on-line for how to check the file against the PGP signature. It's the first time I'm doing something like this; I intend to compile the kernel to make it capable of reading exFAT file-systems.
However, when I want to check the file against the signature, I run into an error. I did as instructed but I get the following:

P.S. I don't have a clue as to what the problem is here, for I have never dealt with PGP personally before. I am not asking about how to make Ubuntu read exFAT, please don't try to go that way and ignoring this PGP hassle. I want to compile the kernel and this is what is standing in the way now, so please help me solve this problem. Thanks for narrowing your answers.