Module not found: Can't resolve 'bootstrap/dist/css/bootstrap-theme.css' in 'C:\react-form-validation-demo\src'

In Bootstrap version 4, the file 'bootstrap-theme.css' have been removed, according to issue in github: https://github.com/twbs/bootstrap/issues/21078, in Change History you get detail

To use version 4 remove the import this, or else downgrade to version 3.

This error is caused by a misconfiguration, version mismatch or corrupted bootstrap install.

If you've got bootstrap and react-bootstrap installed already via some variation of:

npm install --save bootstrap@^4.0.0-alpha.6 react-bootstrap@^0.32.1

(Check if your package.json contains "bootstrap" and "react-bootstrap" if your not sure.)

Just install a different version of bootstrap and rebuild your project. That should replace or add that file (bootstrap/dist/css/bootstrap-theme.css) to that folder. A lower version of bootstrap worked for me as recommended in my create-react-app generated README.md:

npm install --save react-bootstrap bootstrap@3

Adding Bootstrap

You don’t have to use React Bootstrap together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:

Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:

sh npm install --save react-bootstrap bootstrap@3

Alternatively you may use yarn:

sh yarn add react-bootstrap bootstrap@3

reinstall the bootstrap and don't forget to mention --save to the command to save it to the node modules this solved the problem for me.

sudo npm install bootstrap --save