This command generates a unique SHA-512 hash based on the current week number and the kernel name. It could be used to produce a system-specific weekly identifier.
... and in the "Register" web page of the "Arch Linux Forums" - https://bbs.archlinux.org/register.php - if we click on the "Agree" button, I do see that the last question of the user registration form (that then appears) is a specific kind of "Captcha", that has a question like the following:
Your answer
What is the output of "date -u +%V$(uname)|sha1sum|sed 's/\W//g'"?
I think I now understand the confusion: in the CLI, you should type only the command that appears between the double quotes from that question, run it, and write the output in the corresponding text box. So, for my example above, the code that you should type in the CLI to get the corresponding output is the following:
date -u +%V$(uname)|sha1sum|sed 's/\W//g'
So, you would run that command in the CLI, copy the output and paste it into the "Your Answer" text box of the user registration form.
Note that I've noticed that, sometimes, the user registration form uses the "sha512sum" in the middle of the command (as it happened to you when you tried to register) and other times it uses "sha1sum" (as it happened to me when I visited the web page).
When I run date -u +%V$(uname)|sha512sum|sed 's/\W//g' I get the following (it will be different on your machine, as the uname value will be different):