Here's the reproduction:

[hello_test]$ dune init proj hello --ppx ppx_inline_test --inline-tests
Success: initialized project component named hello
[hello_test]$ tree
.
├── _build
│   └── log
└── hello
    ├── bin
    │   ├── dune
    │   └── main.ml
    ├── hello.opam
    ├── lib
    │   └── dune
    └── test
        ├── dune
        └── hello.ml

5 directories, 7 files

The project simply won't build with the dune-project file. And if you put the dune-project file next to the hello.opam file, the _build will be generated automatically when you first start to build, and it will be generated one level down in the hierarchy, in the "hello" directory.

Specifications

• Version of dune (output of dune --version): 2.9.3
• Operating system (distribution and version): Fedora release 35 (Thirty Five)