TL;DR: this error means your numpy install is a mix of numpy 1.x and 2.x files. Uninstall numpy, delete any leftover numpy folders from site-packages by hand, then install numpy fresh. In numpy 2.x the compiled submodule moved from numpy.core to numpy._core, so old leftovers break the import.

## The error
```text
ModuleNotFoundError: No module named 'numpy.core._multiarray_umath'
```

## Fix it
1. Uninstall numpy completely:
   ```
   pip uninstall -y numpy
   ```
   Expected: it reports the package was removed.
2. Check for leftover numpy folders your uninstaller missed:
   ```
   python -c "import site, os; print([p for p in site.getsitepackages()])"
   ```
   Look inside each listed folder for a directory named numpy and delete it. Expected: no numpy folder remains in site-packages.
3. Install numpy fresh:
   ```
   pip install numpy
   ```
   Expected: install completes with no errors.
4. Verify:
   ```
   python -c "import numpy; print(numpy.__version__)"
   ```
   Expected: prints a version like 2.1.3 with no error.

## When this applies
Use this when plain `import numpy` raises ModuleNotFoundError for numpy.core._multiarray_umath, usually right after upgrading numpy or mixing pip and conda installs.

## When it does not apply
If numpy was never installed (plain ModuleNotFoundError: No module named 'numpy'), just install it. If the error mentions a different missing submodule, the mixed-install theory may not hold.

## Compatibility
numpy 2.x on any platform. The old path numpy.core._multiarray_umath existed through numpy 1.26.

## Root cause
NumPy 2.0 renamed the private core package from numpy.core to numpy._core. A partial upgrade leaves 1.x files (which expect the old path) next to 2.x files, and the import machinery cannot find the compiled _multiarray_umath extension at the old location.

## Edge cases
If you use conda, run the same cleanup with conda (conda remove numpy, delete leftovers, conda install numpy). If numpy was installed with --user, also check the user site-packages folder. A virtualenv that is cheaper to recreate can just be rebuilt.