Why cannot I open an .apk file though download "unknown sources" is checked? On my S8 I wanted to install "Blockada" (an app that needs to be sideloaded) but when I click on the .apk to install it says "can't open file".  I have "download from unknown sources" chosen in Settings too!

What is the cause and how do I fix it?
Possible cause can be a corrupted download. Try downloading again. Also try to open in a third party file explorer, many (like ES File Explorer) will let you see package name and version of the apk (without installing) which can help to make sure that the downloaded apk is not corrupted.