export GOPATH="/some/path/here" # only on linux or osx
if on windows adapt above to define environment variable GOPATH to your value
now execute your go build or go run in this same terminal … if this fixes it then you need to save above export command in your system environment variable definition list which is OS specific … above is for linux or osx I do no know the windows equivalent … if linux save above export command in your ~/.bashrc file or in file ~/.profile if on osx