Can the watch lock my computer when I walk away?

While the watch cannot lock it, You can set the mac to auto lock itself after a period of inactivity, by going into System Preferences->Security and Privacy->General->Require Password

It will then lock and require a password after the set time has passed.

Hello,

No this is not possible. Apple did not add this feature, because say you get up real quick to get some water or whatever and your Mac just keeps locking, and it would get annoying. They may also have not added this feature because say a Mac user has their Mac super far away, and uses a keyboard and mouse, since their Mac is far away, then it would lock because the user is far away. So basically this feature is not available. Sorry about that😕