go.mod文件不在root中

I want my project structure like this

repo/
├── app-sv        // my Go api server
│   └── go.mod    // go.mod not working in non-root folder
├── app-cl        // my frontend
├── go.mod        // go.mod is working here
└── README.md

Go modules are not working in a non-root folder, the obvious alternative solution is to create 2 different repo. Is this possible to do? Maybe I'm missing some trivial setup.

EDIT: here's the error