This error message indicates that your project, `babel-preset-react-app`, is importing a package named `@babel/plugin-proposal-private-property-in-object` without declaring it in the devDependencies. This is currently working because the package is already in your `node_modules` folder for unrelated reasons, but it may break at any time. To work around this error, you can add `@babel/plugin-proposal-private-property-in-object` to your `devDependencies` [1][2].

Here's an example of how to add the package to your `devDependencies` in your `package.json` file:

{
  "name": "my-project",
  "version": "1.0.0",
  "devDependencies": {
    "@babel/plugin-proposal-private-property-in-object": "*"
  }
}

Alternatively,You can do this by running the following command in your project directory:

npm install --save-dev @babel/plugin-proposal-private-property-in-object

or with yarn:

yarn add --dev @babel/plugin-proposal-private-property-in-object

This will add the `@babel/plugin-proposal-private-property-in-object` package as a development dependency of your project. You can then run `npm install` or `yarn install` to install the package and its dependencies.

It's worth noting that `babel-preset-react-app` is part of the `create-react-app` project, which is no longer maintained. If you're starting a new project, you may want to consider using maintained alternatives such as Vite instead [1].