"package XXX is not in GOROOT" when building Golang project

A pretty dumb conclusion (mostly on my part) but my issue came from having done go mod init in each of the folders. after removing go.mod and go.dep from each of the folders I did go mod init in, I could build without issue (through terminal)

Also, my packages in GoLand were not being detected because I had Go Modules enabled in the Settings. I disabled it and GoLand was able to index the external packages and my own packages.

To anyone who does want modules to work with GoLand after they have stopped doing so, make sure 'Enable Go modules integration' is checked in the Preferences as such: