Eclipse Java IDE JUnit5: junit.jupiter.api.Assertions is not accessible

I'm new to the whole programming stuff but here's my problem:

I used to add my JUnit test cases in Eclipse by right clicking on the project, and just add New > JUnit Test Case.

Currently, I am not able to implement any test methods because Eclipse tells me on the line

import static org.junit.jupiter.api.Assertions.*;

the error message

The type org.junit.jupiter.api.Assertions is not accessible.

Error I get in the IDE:

I tried the following:

1. Reinstalling Eclipse, using a fresh workplace.
2. Adding the JUnit to Build path

Nothing helped.

It worked and works in older projects just fine.

Here is how the Package Explorer looks:

What am I missing?

You use the Java Platform Module System (JPMS) by having a module-info.java file in the default package probably without the required requires <module>; statement. JPMS was introduced in Java 9.

Do one of the following:

• Delete the module-info.java file (if needed, you can recreate it via right-clicking the project folder and choosing Configure > Create module-info.java)
• In module-info.java add the corresponding requires statement, e.g. by going to the line with the import statement and using the corresponding Quick Fix (Ctrl+1)