…there is a typographic error here. In Linux (or any *NIX) the file “.” is “the current directory” (and two dots is “the parent directory of the current directory”, but the forum is forcing three dots “..”). When you specify “/” instead of “./” you change this from “the file at the current directory” to “the file in /, the root of all directories”. So be careful to include the “.”:
sudo dpkg -i ./sdkmanager_1.1.0-6343_amd64.deb
Note: “./something” implies “something of the current directory”.