Please provide all the files, including Go files and go.mod, to reproduce this issue. Also provide exactly what commands you ran to encounter the error.

CC @thepudds @myitcv

Hmm. I just ran through that "Quick Start Example" by copy/pasting from the wiki, and it seemed to work.

Prep by cleaning slate:

rm -rf /tmp/scratchpad/hello

Commands copied from the wiki:

mkdir -p /tmp/scratchpad/hello
cd /tmp/scratchpad/hello
go mod init github.com/you/hello
cat <<EOF > hello.go
package main

import (
    "fmt"
    "rsc.io/quote"
)

func main() {
    fmt.Println(quote.Hello())
}
EOF

go build 
./hello

Output:

Hello, world.

And here is the resulting go.mod:

$ cat go.mod

module github.com/you/hello

require rsc.io/quote v1.5.2

@horseboxer In addition to the questions @mvdan asked, what do you see if you do go get -v rsc.io/quote from within that directory /tmp/scratchpad/hello?

@horseboxer just wanted to check in to see if you were able to look at this a bit more? (For example, see the immediately prior comment as well as the suggestions from @mvdan).

Note that go get -v rsc.io/quote will likely tell you more information than go build alone. For example, it can be that go build will give a very general error you saw like cannot find module for path rsc.io/quote without any additional detail, but then running go get -v rsc.io/quote will often give a more specific error (which can be more helpful than the initial error from go build).

Turns out freshly installed Ubuntu VMs don't have git installed :-(. Fixed the user-error and it works now. Sorry to waste your time.

I think cmd/go should give a better error message here. I can reproduce OP's confusing error with a few commands inside a Docker image:

$ cat repro.sh
docker run -i golang:1.11.3-alpine <<-SCRIPT
        export GO111MODULE=on
        git version && { echo "git needs to be missing"; exit 1; }
        mkdir /foo && cd /foo
        go mod init test.tld/foo
        go env GOMOD
        cat >f.go <<-FILE
                package main
                import _ "rsc.io/quote"
                func main() {}
        FILE
        go build
SCRIPT
$ sh repro.sh
/bin/sh: git: not found
go: creating new go.mod: module test.tld/foo
/foo/go.mod
build test.tld/foo: cannot find module for path rsc.io/quote

@bcmills note that this only happens with a package main. With package foo, I get:

f.go:5:1: unknown import path "rsc.io/quote": git init --bare in /go/pkg/mod/cache/vcs/4db0c9594744360b0eaa452d2ccfbd45b05dffb9810882957d10d69e61e66382: exec: "git": exec
utable file not found in $PATH

@bcmills for what it's worth, I can't reproduce the issue on go version devel +9ed9df6ca2 Wed Dec 19 00:13:22 2018 +0000 linux/amd64, which should be pretty close to the first beta.

Frustratingly enough, there's no 1.12 docker image at the moment, so I can't be 100% sure. But locally, PATH=/home/mvdan/tip/bin gotip build gives a good error, while PATH=/home/mvdan/tip/bin go1.11.3 build gives the confusing error. So this might be already fixed in master.

@mvdan, thanks for investigating in detail!

My results match yours: I don't know why, but this seems to be fixed in 1.12.

$ PATH=~/bin go1.11.4 build .
build golang.org/issue/scratch: cannot find module for path rsc.io/quote

$ PATH=~/bin go1.12beta1 build .
build golang.org/issue/scratch: cannot load rsc.io/quote: git init --bare in /tmp/tmp.BJQhURaAuR/_gopath/pkg/mod/cache/vcs/4db0c9594744360b0eaa452d2ccfbd45b05dffb9810882957d10d69e61e66382: exec: "git": executable file not found in $PATH

@bcmills perhaps we should add a regression test, particularly since noone seems to know how we fixed this :)

A regression test does seem like a good idea.

I can submit my reproducer above as a test script by tomorrow.

Change https://golang.org/cl/155537 mentions this issue: cmd/go: add regresion test for cryptic vcs errors