I (and many others) always had problems making JUnit (as added by Eclipse automatically when creating JUnit test cases) work with modular projects, i.e. projects that use module-info.java files to define dependencies. Finally, I found solutions:

• Let the new project wizard not create the module-info.java file -- deleting it afterwards might not be enough as Eclipse did already some modification the the module path settings (OK, trivial) or
• Choose Java ≤8 in settings (i.e. module-info.java ignored -- again: trivial) or
• Apply quick-fixes: in the class containing your JUnit test cases, hover over the org.junit.jupiter.api import and select the quick-fix: “Add ‘requires […]’ to module-info.java”. Then in module-info.java: hover with mouse over the squiggle line (the important point is: clicking on the light bulb does not give any quick-fix, so you need to hover) and do: “Move classpath entry ‘JUnit5’ to modulepath”. This should fix it! or
• Create an Eclipse project with extra src folder (e.g. src-test or use the Maven default structure) that has (via “Allow output folders for source folders”) its own output folder (e.g. bin-test or use the Maven default structure) and that has “Contains test sources” toggled to “Yes” (in project properties - Java Build Path -Source). The test src folder should then have a more grey-ish icon. Either do this with the New project wizard, or afterwards using project properties. As a result, JUnit is then not part of the modular project anymore. (Has also the advantage that test code is better separated.)