The JavaFX package is not included in JDK 9 and later. You need to install JDK 8, or you can add the JavaFX package separately, for example, from JDK 8 (jfxrt.jar).

Try this solution in IDEA press ctrl+shift+alt+s (File -> Project Structure) then select Project -> Project Language Level and select 8 or above.

If this happens to you in a modular project with JavaFx version 17 using Maven, just try the usual things like standing on your head, walking on the ceiling, rotating the Earth in the opposite direction or even reading https://openjfx.io/openjfx-docs/ and then just switch to the version 17.0.1 or later and it should work:

<dependency>
    <groupId>org.openjfx</groupId>
    <artifactId>javafx-controls</artifactId>
    <version>17.0.1</version>
</dependency>

If you are java 8+ the javafx libs are not included. you need to add them via your favorite build tool as a compile runtime lib.

Step 1: Configuring everything to java 9

First you need to make sure everything is configured to java 9. Click on File->Project Structure

• On the project menu you have the Project SDK and the language level (Image)

• And on the modules menu you have to check 2 tabs (you probably have to to it for every module):

  • Souces has got the language level (Image)

  • And dependences has got the Module SDK (Image)

Step 2 Rebuilding your project

Then you need to rebuild your project:

Right-click on your project and click on Rebuild Module (Image)

Then just wait until your project is rebuilt and try to run it again.

My problem was that I deleted my SDK 15 and installed SDK 16. Check your Libraries locations under File -> Project Structure -> Libraries. If your library version is lower than what you have selected under Project Settings -> Project -> Project SDK. You are going to have a bad time.

I've reproduced the same issue after adding the library, but using Java 11 & JavaFX 11.

To fix it, I've followed such steps:

1. Firstly, click Project Structure -> Libraries and check if library isn't used:

2. Secondly, open Project Structure -> Problems and click [Fix] -> Add to Dependencies...:

3. Thirdly, choose as module and click OK:

4. Lastly, click Apply and OK.

As result, will be: