After go update (1.8 -> 1.9) on Visual Studio Code, you may have a message like “Go: cannot find GOROOT directory”, you have to update all tools.

You can check the issue on vscode-go github

To fix the issue exec that on a shell

gocode close

go get -u -v github.com/nsf/gocode
go get -u -v github.com/rogpeppe/godef
go get -u -v github.com/zmb3/gogetdoc
go get -u -v github.com/golang/lint/golint
go get -u -v github.com/ramya-rao-a/go-outline
go get -u -v sourcegraph.com/sqs/goreturns
go get -u -v golang.org/x/tools/cmd/gorename
go get -u -v github.com/tpng/gopkgs
go get -u -v github.com/acroca/go-symbols
go get -u -v golang.org/x/tools/cmd/guru
go get -u -v github.com/cweill/gotests/...
go get -u -v golang.org/x/tools/cmd/godoc
go get -u -v github.com/fatih/gomodifytags
go get -u -v github.com/josharian/impl