Go: 'go: go.mod file not found in current directory or any parent directory' or 'go: cannot find main module'. How do I set up the module correctly?

Cause: since Go 1.16, the go command works in module mode by default. Commands like go build, go run and go test look for a go.mod file in the current directory or its parents to know which module you're in. The error means there isn't one: a new project that was never initialized, running the command from the wrong directory (for example the parent of the project), code copied from an old GOPATH-style tutorial, or a monorepo where the module lives in a subfolder. Fix: 1. For a new project, create the module in the project's root directory: go mod init example.com/yourname/project (use your repository path, e.g. github.com/you/project, if you'll publish it). Then run go mod tidy to add the dependencies your imports need and create go.sum. 2. If the project already has a go.mod somewhere, cd into that directory (or a subdirectory of it) before running go commands. In a repository with several modules, each module directory needs its own go.mod; a go.work file (go work init ./a ./b) lets you work on several at once. 3. To install a tool rather than build your own code, use go install example.com/tool@latest, which doesn't need a go.mod in the current directory. 4. Don't switch modules off with GO111MODULE=off to make old tutorials work; GOPATH mode is legacy. Convert the project with go mod init instead. 5. In CI and Docker builds, make sure the working directory contains go.mod (set WORKDIR and copy go.mod/go.sum before running go mod download). Verify: go env GOMOD prints the path to your go.mod (not /dev/null or empty), and go build ./... succeeds. Sources: https://go.dev/ref/mod https://go.dev/doc/tutorial/create-module