Just wanted to note I had this issue and adding this fixed the error for me:

/** @jsx jsx */
import { jsx } from '@emotion/core';

Just to clarify, you have to add /** @jsx jsx */; at the top of the file.

This method solved my problem:

/** @jsxImportSource @emotion/react */
import { css } from "@emotion/react"

export default function somepage() {

  const color = 'red'

  return (<div
    css={css`
      background-color: hotpink;
      &:hover {
        color: ${color};
      }
    `}
  >
    Hello World.
  </div>)
}

What Does this do?

If you are using a zero-config tool with automatic detection of which runtime (classic vs. automatic) should be used and you are already using a React version that has the new JSX runtimes (hence runtime: 'automatic' being configured automatically for you) such as Create React App 4 then /** @jsx jsx */ pragma might not work and you should use /** @jsxImportSource @emotion/react */ instead.

As detailed in the Emotion documentation, use of the css prop requires you to replace React with jsx as the target function for JSX elements (known as the “pragma”), which will enable Emotion to intercept the css prop and transform it into a className prop for React.

For example:

<p>Hi</p>

// Becomes the following by default:
React.createElement("p", {}, "Hi")

// But with the `jsx` pragma, it becomes:
jsx("p", {}, "Hi")

There are two outlined approaches to achieve this. You only need to select one. If you are able to configure Babel in your application, the first is the recommended approach, but either one works fine:

1. Install a Babel plugin that will configure jsx as the default handler for all code in your app

   The most direct way to do this is by adding the relevant Babel preset to your Babel configuration like so:

   // Option 1A — Good
   // Add @emotion/babel-preset-css-prop to your dev dependencies, then
   // add the preset to your .babelrc configuration:
   
   {
     "presets": ["@emotion/babel-preset-css-prop"]
   }
   

   If you are able to do this, though, I recommend instead configuring babel-plugin-emotion, which includes the css prop configuration as well other features like minification, dead code elimination, and hoisting:

   // Option 1B — Better
   // Add babel-plugin-emotion to your dev dependencies, then add
   // the plugin to your Babel configuration (e.g. .babelrc)
   {
     "plugins": ["emotion"]
   }
   

2. Import the jsx function from Emotion and instruct Babel to use this imported function on a per-file basis using pragma

   /** @jsx jsx */
   import { jsx } from '@emotion/core'
   

You need to install this babel plugin, babel-preset-css-prop

In addition to include the @emotion/babel-plugin, there was some configuration that I have missed that created that error.

{
  "presets": [
    [
      "@babel/preset-react",
      { "runtime": "automatic", "importSource": "@emotion/react" }
    ]
  ],
  "plugins": ["@emotion/babel-plugin"]
}

The "importSource": "@emotion/react" was missing. After adding it, the error was gone and the styles were implemented correc5y.

There are 2 ways to get started with the css prop.

1. Babel Preset
2. JSX Pragma

Both methods result in the same compiled code.

JSX Pragma

Set the jsx pragma at the top of your source file that uses the css prop. This option works best for testing out the css prop feature or in projects where the babel configuration is not configurable (create-react-app, codesandbox, etc.).

/** @jsx jsx */

if it didn't work use:

/** @jsxImportSource @emotion/react */

Babel Preset

This method will not work with Create React App or other projects that do not allow custom Babel configurations. Use the JSX Pragma method instead.

.babelrc

{
  "presets": ["@emotion/babel-preset-css-prop"]
}

Couldn't solve your problem? Consider reading official documentation: https://emotion.sh/docs/css-prop

If you're using Vite, dont forget to add following setting in your vite.config.ts :

export default defineConfig({
  plugins: [
    react({
      jsxImportSource: '@emotion/react',
    }),
  ]
})

It allowed me to fix that issue.