I've made a homebrew magical sword that requires attunement, checked the box for it, and written in a description. When I add  it to a  character, though, all the bonuses apply automatically, and it's not shown in the list of items I could attune to even if I wanted. How can I make this requirement enforceable?

Try this:

Resave the whole homebrewer item. That will trigger the system to pick up the most current version of your item.

Then log out and log back in. That will trigger the server to clear its internal cache.

Then wait 20 minutes to give the system time for everything to update.