If you installed Disco by extracting the binary from a pre-built package, make sure that you placed the binary in a directory on your $PATH. On Linux/macOS you can confirm this with which disco.
If you installed Disco via go install, make sure that you precisely follow the Go installation instructions for your platform.
Specifically, ensure that the $GOPATH/bin directory is part of your $PATH. For example, you might want to add this to your shell's initialization file: export PATH=$(go env GOPATH)/bin:$PATH. See this article for more information.