go: cannot find main module; see ‘go help modules’
I tried GO111MODULE=auto and off, got same error.
I read the release notes, and do not see anything directly related (might have missed something though).
If I run go mod tidy then go install it works. I understand we are moving to modules, but it seems the release notes are actually lacking the part saying its not optional (unless I am wrong).
I followed the Go 1.16 Release Notes instructions. I am unable to reproduce your issue.
Go 1.16 Release Notes
Modules
Module-aware mode is enabled by default, regardless of whether a go.mod file is present in the current working directory or a parent directory. More precisely, the GO111MODULE environment variable now defaults to on. To switch to the previous behavior, set GO111MODULE to auto.
$ dir
main.go
$ cat main.go
package main
func main(){}
$ go1.16 version
go version go1.16 linux/amd64
$ go1.16 env GO111MODULE
on
$ go1.16 install
go: cannot find main module; see 'go help modules'
$ export GO111MODULE=auto && go1.16 env GO111MODULE
auto
$ go1.16 install
$ export GO111MODULE=off && go1.16 env GO111MODULE
off
$ go1.16 install
$ export GO111MODULE=on && go1.16 env GO111MODULE
on
$ go1.16 install
go: cannot find main module; see 'go help modules'
$