"Why can't I empty my Trash, no matter what I try?"
If you can't empty your Trash, and you've tried every suggestion you can find online, plus everything else you can imagine, chances are there are files in it subject to System Integrity Protection (SIP). As long as MacOS is guarding those files, there's not a thing you can do. (One way to identify such files is to check their permissions, which will show as "unknown.") Luckily, once you know what's happening, the fix isn't hard -- you just need to temporarily turn off SIP. To do that, boot into Recovery, start Terminal from the Utilities menu, and enter csrutil disable Restart to boot back into MacOS normally, then empty the Trash. Then return to Recovery and restore SIP with csrutil enable That should do it. And next time, maybe avoid putting anything in the Trash that you shouldn't be messing with in the first place! (That's me speaking from experience, after trying to delete stuff from /var/folders/. Grrrrrr.)