It looks like git may not be in your PATH environment variable.
golang "go get" command showing "go: missing Git command" error - Stack Overflow
"The go get fetching of source code is done by using one of the following tools expected to be found on your system either git, svn, hg.
Install git from this link https://git-scm.com/downloads
After installing git you should navigate to the environment variables setting and add the path of git.exe(executable file) which is found in the bin. So the path should look like this "C:\Program Files\Git\bin". Restart your IDE and the command should be working."