Code:
fastboot --help
usage: fastboot [ <option> ] <command>
commands:
...
flash <partition> [ <filename> ] flash downloaded data
...
partitions:
bootloader bootloader, 376KiB
prod overlaps bootloader, 120KiB
bist bist, 256KiB
userdata userdata, 5KiB
userpartition userpartition
mbr master boot record
kernel primary kernel
diags_kernel diags kernel
system main system (root) partition
diags secondary system (diags) partition
data user data
fastboot flash diags_kernel kt_5.0.0-diags_kernel (from memory,
CHECK THAT, I just did)
You are missing a target alias. This is a bad tool to use without understanding. It can do very bad things...
(you asked it - with your malformed command - to write from STDIN to an unknown target partition called kt_5.0.0-diags_kernel from what I can make of the error, thanks goodness you dont have one called that ; ) )
try
fastboot --help - worth a read. For sure.